android: implement support for proxies

*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