[MM-53457] Standalone client

[streamer45]

Summary

PR implements a standalone Golang client for Calls. This is mostly based of the official one included in the plugin. The implementation abstracts away all the logic necessary to successfully join a call given a SiteURL, AuthToken and ChannelID.

This includes:

• WebSocket client implementation.
• WebRTC client implementation.
• Any Calls plugin specific API logic (e.g. join message, reconnect support, etc).

There are however some key differences:

• The WebRTC API is the one provided by pion which comes with its own set of limitations (e.g. perfect negotiation is not yet supported).
• The client is currently receive-only, purely useful for receiving tracks since that's the main use case. I don't exclude extending this to be a full functional client but not a priority. If we ever do that it may be interesting to replace the one used for load-testing with this.

This also comes with some additional limitations and potential TODOs:

• We are not reading the plugin's config which means we are not setting any ICE server configs. As the client is mostly meant to connect internally they shouldn't be necessary but something that could be added to make it a little closer to a real implementation.
• We are not implementing the MM reliable websocket functionality. This is partly because it doesn't really work in HA clusters so not sure if it's worth adding for this use case.

To note, this PR also includes a very basic e2e pipeline as tests are connecting against a running version of the plugin compiled with the latest rtcd changes. This could also serve as a good base to implement more tests that check general functionality.

Ticket Link

https://mattermost.atlassian.net/browse/MM-53457

[cpoile]

Looks very clean, nicely done. Nothing jumps out as a need to fix, and I need to get more familiar with pion to know if you're doing anything out of the ordinary there. But I'll assume you're not. 👍

[streamer45]

Added a couple of extra commits to facilitate the transcription use case, mainly being able to react to user disconnecting and call ending.

[cpoile]

Nice, non-blocking comments

[cpoile]

Thanks!