Integration tests in CI

[alan-george-lk]

This PR does the following:

• Moves all testing into a new tests.yml CI stage
• New more specific/performance improved runners for Windows/Mac
• Adds the ability to run integration tests in CI via a locally-installed LiveKit server (Linux only)
• Adjusts unstable integration tests to improve stability
  • PublishesAndReceivesFramesEndToEnd -> Adjusted to more closely match the Rust SDK layout
  • Timestamp test -> Matches the E2E TypeScript test by re-using the same timestamp throughout
• A more explicit parallel jobs function for improving CI times (has helped slightly on Windows, for example)
• Consolidates 5 different prior env vars for integration/stress tests down to LIVEKIT_TOKEN_A/B
• Changes Rust cache action to match Rust SDK
• Add timeouts to testing stages to catch deadlock bugs
• Disables GMock to save time on build (not used)

Some caveats:

• Still runs unit tests on all prior platforms
• Mac runner performance is not passing on C++ integration tests through FFI. Internal ticket was created to further debug this

[stephen-derosa]

sweet looks good to me thanks for doing! Consider waiting on this until the clang format stuff is in to make a smaller diff in that MR

[stephen-derosa]

LGTM -- great work, really excited to have this in!