Managed gateway groundwork

[johanstokking]

Summary

References https://github.com/TheThingsIndustries/lorawan-stack/issues/4256
Groundwork for #7178

This adds groundwork for managed gateways in The Things Stack.

Changes

• Add groundwork for TTI gateway protocol as GS frontend
• Add The Things Gateway Controller client and integration in Gateway Configuration Server
• Add events multiplexer to add additional event streams, in this case managed gateway events that come from The Things Gateway Controller
• Add missing checks for gateway tokens
• Resolve some technical debt, including but not limited to
  • Existing WebSocket listeners in GS
  • Lint warnings
  • Obsolete settings
  • Structure of GCS
  • Missing GS IO test cases and values (suppressed for UDP and LBS LNS that have limited support for this)

Testing

Testing of this PR is mostly through unit tests and tedious manual tests with WebSockets and TTS API calls.

Testing steps for QA sign off will be part of #7178.

Notes for Reviewers

@KrishnaIyer is the main reviewer.

This is draft until:

• Testing section is filled out
• GS frontend is implemented and tested
• More integration testing is done
• Commit trail is clean
• Infrastructure PR is (almost) ready
• Changelog is updated
• CI pass

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI, according to the compatibility commitments in README.md for the chosen target branch.
• Documentation: Relevant documentation is added or updated.
• Testing: The steps/process to test this feature are clearly explained including testing for regressions.
• Infrastructure: If infrastructural changes (e.g., new RPC, configuration) are needed, a separate issue is created in the infrastructural repositories.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.

[KrishnaIyer]

Reviewed API, GCS and the events mux. Looks good to me.