Outbound settings for chain-proxying

[TPS]

As mentioned here & preceding, outbound proxy settings are necessary for those of us who'd like to use AG in conjunction with another proxy, e.g., Orbot in that mode.

[ameshkov]

We should implement it as a low-level setting.

Name: pref.http.proxy.upstream
Type: String
Example: 127.0.0.1:8888

Proxifying plain HTTP is rather straightforward. But I don't yet understand how we will proxify HTTPS traffic. Simple solution is to stick with HTTPS tunneling. But in this case we may loose the ability to decrypt & filter it.

[ameshkov]

One more user request: add PAC support