[WIP] [nextest-runner] initial support for recording runs

[sunshowers]

Gated behind an environment variable for now.

TODO:

• Code to read test runs (probably in a separate PR)
• Tests
• Docs
• Factor out code to handle experimental features (probably in a separate PR)
• Migrate store.dir to {target_dir}/nextest (stabilization is blocked on this)
• Remove write_meta, replace with RunStarted or another meta event.
• Factor out global_store_dir.

[codecov]

Codecov Report

Attention: 856 lines in your changes are missing coverage. Please review.

Comparison is base (92522ce) 77.84% compared to head (a50803c) 74.34%.

Files	Patch %	Lines
nextest-runner/src/run_store.rs	0.00%	734 Missing ⚠️
nextest-runner/src/reporter/structured/recorder.rs	0.00%	67 Missing ⚠️
cargo-nextest/src/dispatch.rs	48.38%	16 Missing ⚠️
nextest-runner/src/reporter/structured.rs	61.53%	10 Missing ⚠️
nextest-runner/src/runner.rs	28.57%	10 Missing ⚠️
nextest-runner/src/config/config_impl.rs	88.09%	5 Missing ⚠️
nextest-runner/src/config/scripts.rs	0.00%	4 Missing ⚠️
cargo-nextest/src/errors.rs	0.00%	3 Missing ⚠️
nextest-runner/src/errors.rs	0.00%	3 Missing ⚠️
nextest-runner/src/time/stopwatch.rs	0.00%	3 Missing ⚠️
... and 1 more

Additional details and impacted files

@@ Coverage Diff @@ ## main #1265 +/- ## ========================================== - Coverage 77.84% 74.34% -3.51%  ========================================== Files 72 74 +2 Lines 17656 18571 +915 ========================================== + Hits 13744 13806 +62  - Misses 3912 4765 +853 

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