android: implement support for proxies

[Augustyniak]

Note: even though this ticket is for Android specifically a lot of work that's going to end up being referenced in here will end up being shared between iOS and Android implementations for supporting HTTP proxies.

Upstream Envoy

• A GH issue for tracking the initial work for supporting proxies in the upstream Envoy https://github.com/envoyproxy/envoy-mobile/issues/1622
• https://github.com/envoyproxy/envoy/pull/21942
• https://github.com/envoyproxy/envoy/pull/21593
• https://github.com/envoyproxy/envoy/pull/22977
• https://github.com/envoyproxy/envoy/pull/23002
• https://github.com/envoyproxy/envoy/pull/23079
• https://github.com/envoyproxy/envoy/pull/23077

Envoy Mobile

Minimum required to test proxies with HTTP and HTTP/2 traffic

• https://github.com/envoyproxy/envoy-mobile/pull/2416
• https://github.com/envoyproxy/envoy-mobile/issues/2531
• https://github.com/envoyproxy/envoy-mobile/issues/2532

Should have

• https://github.com/envoyproxy/envoy-mobile/issues/2547
• https://github.com/envoyproxy/envoy-mobile/issues/2556

Other

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