HTTP CONNECT proxy support

[cretz]

What was changed

• Added new client::proxy module for HTTP CONNECT support
  • Hyper/Tonic don't support this natively, so some workarounds had to be done
  • Based on user need, this also includes authentication
• Added new http_connect_proxy client option

Can't really perform a test of this without a more involved integration test that has a full proxy server implementation. I have manually tested this with Python SDK using this version of core and connecting to Temporal cloud through the proxy. Now that proxies are officially supported by Temporal, there's a feature PR at https://github.com/temporalio/features/pull/447 for testing them, and Python will be added once this is merged and the Python SDK is updated

Checklist

1. Closes #309

[cretz]

Looks like this was fairly straightforward, nice.

The result was fairly straightforward but this took a lot of time because I had to dig through the internals because Tonic/Hyper don't make this easy. They don't make it easy to change the target TCP stream and they don't make it easy to do a URL call + upgrade after TCP connection. I also had to setup proxies to test with and verify this behavior matches existing Go and Java gRPC behavior. But yes, luckily the amount of code needed is small.

(waiting on external CI fix before merging)