ci: optimize ci setup

[fubhy]

For this pull-request:

• Switch to nextest as test runner
• Configure mutex via nextest filtersets for tests that do not support concurrency (some unit tests and most integration/runner tests)
• Use justfile to create more accessible aliases for common test & development commands. Use them in CI.
• Use beefier multicore CI runners instead of the github default runners

For follow-up pull request:

• Explore how we can refactor integration & runner tests to actually treat test cases as individual tests instead of implementing a custom test runner internally
• Create and use a pre-built container image for the CI runner that comes with preinstalled system dependencies (protoc & just) to save on install time (~2 minutes per CI job)
• Benchmark test suite and identify slow tests and whether there are any low hanging fruits for optimisation