ios proxy: implement support for proxies on iOS

[Augustyniak]

This is supposed to be a parent ticket for all work related to adding support for proxies on iOS.

Must have tickets:

• ios proxy: add support for proxies configured using IP address and hostname #2606
• ios proxy: add support for PAC proxies #2615

Should:

• ios,c++: add a way to enable proxy support to c++ engine builder #2616

To be determined:

• ios/android proxy: support proxy with auth #2574