feature: add Spring Boot 2.4 config data loader support

[vdiskg]

What's the purpose of this PR

1. add Spring Boot 2.4 config data loader support

2. make apollo client system property names valid and add warn to the deprecated property names

see the unit test
ApolloClientSystemPropertyInitializerTest#testSystemPropertyNames

see the warn
com.ctrip.framework.apollo.util.ConfigUtil#getDeprecatedCustomizedCacheRoot
com.ctrip.framework.foundation.internals.provider.DefaultApplicationProvider#initDeprecatedAccessKey
com.ctrip.framework.apollo.internals.ConfigServiceLocator#getDeprecatedCustomizedConfigService

2.1 property names changed:

apollo.cacheDir -> apollo.cache-dir
apollo.accesskey.secret -> apollo.access-key.secret
apollo.configService -> apollo.config-service

2.2 environment variables names changed:

APOLLO_CACHEDIR -> APOLLO_CACHE_DIR
APOLLO_ACCESSKEY_SECRET -> APOLLO_ACCESS_KEY_SECRET
APOLLO_CONFIGSERVICE -> APOLLO_CONFIG_SERVICE

3. add webClient extension for apollo client

4. add extension messaging type long_polling, websocket

the default value is apollo.client.extension.messaging-type=long_polling and the websocket is just a empty Implementation
as a placeholder, and will complete in a future version

Which issue(s) this PR fixes:

Fixes #3697

Instruction

1. Basic Functions

1.1 add the dependency

        <dependency>
            <groupId>com.ctrip.framework.apollo</groupId>
            <artifactId>apollo-client-config-data</artifactId>
        </dependency>

1.2 configure the app.id, env, apollo.meta same way as before

1.3 configure spring.config.import on application.properties or application.yml

using default namespace application

# old way
# apollo.bootstrap.enabled=true
# not configure apollo.bootstrap.namespaces
# new way
spring.config.import=apollo://

or

# old way
# apollo.bootstrap.enabled=true
# apollo.bootstrap.namespaces=application
# new way
spring.config.import=apollo://application

using custom namespace

# old way
# apollo.bootstrap.enabled=true
# apollo.bootstrap.namespaces=your-namespace
# new way
spring.config.import=apollo://your-namespace

using multi namespaces
note: please put upside down of your multi namespaces.

# old way
# apollo.bootstrap.enabled=true
# apollo.bootstrap.namespaces=namespace1,namespace2,namespace3
# new way
spring.config.import=apollo://namespace3, apollo://namespace2, apollo://namespace1

2. webClient with spi

2.1 webClient dependency note

webClient can based on reactor netty httpclient, jetty reactive httpclient or apache httpclient5. the necessary dependency are as below

2.1.1 reactor netty httpclient

        <!-- webclient -->
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-webflux</artifactId>
        </dependency>
        <!-- reactor netty httpclient -->
        <dependency>
            <groupId>io.projectreactor.netty</groupId>
            <artifactId>reactor-netty-http</artifactId>
        </dependency>

2.1.2 jetty reactive httpclient

        <!-- webclient -->
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-webflux</artifactId>
        </dependency>
        <!-- jetty reactive httpclient -->
        <dependency>
            <groupId>org.eclipse.jetty</groupId>
            <artifactId>jetty-reactive-httpclient</artifactId>
        </dependency>

2.1.3 apache httpclient5

        <!-- webclient -->
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-webflux</artifactId>
        </dependency>
        <!-- apache httpclient5 -->
        <dependency>
            <groupId>org.apache.httpcomponents.client5</groupId>
            <artifactId>httpclient5</artifactId>
            <version>5.1</version>
        </dependency>
        <dependency>
            <groupId>org.apache.httpcomponents.core5</groupId>
            <artifactId>httpcore5-reactive</artifactId>
            <version>5.1</version>
        </dependency>

2.2 add the dependency

        <dependency>
            <groupId>com.ctrip.framework.apollo</groupId>
            <artifactId>apollo-client-config-data</artifactId>
        </dependency>
        <!-- webclient -->
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-webflux</artifactId>
        </dependency>
        <!-- reactor netty httpclient as an example here, both jetty reactive httpclient or apache httpclient5 are ok -->
        <dependency>
            <groupId>io.projectreactor.netty</groupId>
            <artifactId>reactor-netty-http</artifactId>
        </dependency>

2.3 configure the app.id, env, apollo.meta same way as before

2.4 configure spring.config.import and other necessary properties on application.properties or application.yml

spring.config.import=apollo://application
apollo.client.extension.enabled=true

2.5 provide a spi Implementation of the interface com.ctrip.framework.apollo.config.data.extension.webclient.customizer.spi.ApolloClientWebClientCustomizerFactory

[nobodyiam]

This is a great feature and it looks good to me.
However, since it adds a new sdk so I hope it could be reviewed by more committers.

[kezhenxu94]

Create feature, thanks!

I suggest not to mix renaming property / env var names and new feature in a single pull request next time.

[vdiskg]

Create feature, thanks!

I suggest not to mix renaming property / env var names and new feature in a single pull request next time.

renaming property is strong related to the new feature.
see the
org.springframework.boot.context.properties.bind.Binder#bind
org.springframework.boot.context.properties.source.ConfigurationPropertyName#isValid
if a property name is not valid, the method bind which is used in com.ctrip.framework.apollo.config.data.system.ApolloClientSystemPropertyInitializer will throw an InvalidConfigurationPropertyNameException.

[Anilople]

Great feature!

I am not familiar with spring.config.import functionality so cannot give more help.

[nobodyiam]

LGTM