Aggregated Watches

This change set introduces a new endpoint to the Kubernetes API proxy. The API is located a <prefix>/watch and implements a new protocol on top of a WebSocket communication.

The endpoint can be used to aggregated multiple Kubernetes Watch API requests into a single WebSocket connection instead of having to handle multiple long-lived plain HTTP connections.

The protocol is named gitlab-agent-watch-api and documented in doc/watch-aggregator-api.md.

The intended user for this endpoint is the Dashboard for Kubernetes inside the GitLab UI.

Closes #541 (closed)

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Open Points:

• Tests
• Consider defining and using a WebSocket subprotocol
• Documentation of custom protocol
• Cleanup here and there, but nothing major
• Why the hell does proto-gen NOT put the json_name into the json tag ?! (see watch id in WatchRequest)
• (optional) complete request with other fields
• Write follow-up issue to include new metric for watches into our Grafana dashboard
  • Issue: Add aggregated watch metrics to Grafana dashboards (#602)
• Agentk won't shutdown when watch is open
• Open issue in websocket package for cancelation with cause handling
  • Issue: https://github.com/nhooyr/websocket/issues/454