feat: add custom hyper connector support

[Dreaming-Codes]

Summary

• Add custom-hyper-connector feature flag that allows users to provide their own hyper-compatible connector
• Enables use cases like WireGuard tunnels, custom TLS implementations, and network virtualization (e.g., smoltcp)
• Includes two examples: custom_connector.rs (basic) and tor_arti.rs (Arti Tor client integration)

Changes

• New ClientBuilder::custom_connector() method accepting any Service<Uri> that returns a connection implementing Read + Write + Connection
• New connector module exporting types needed for custom connector implementations
• Examples demonstrating custom TCP/TLS connectors and Arti Tor integration

Closes #1535
Closes #2633
Closes #1321

[Dreaming-Codes]

Another example is also available here https://github.com/Dreaming-Codes/wireguard-hyper-connector/blob/master/examples/reqwest_custom_connector.rs

[Dreaming-Codes]

@seanmonstar Anything missing to merge this?

[Daanoz]

I have just tried this branch for my use case: an mTLS connection with the private key in an HSM only accessible via a PKCS#11 URI using an OpenSSL Provider.
I already had created a Hyper Connector that supports this, and I could directly use it's implementation with your branch without modifications!

I would love to be able to use reqwest for our applications, happy to help with any validation work if needed!