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:

• https://github.com/envoyproxy/envoy-mobile/issues/2606
• https://github.com/envoyproxy/envoy-mobile/issues/2615

Should:

• https://github.com/envoyproxy/envoy-mobile/issues/2616

To be determined:

• https://github.com/envoyproxy/envoy-mobile/issues/2574