[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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