[nextest-runner] add support for setup scripts

[sunshowers]

This is pretty large! Hoping to get this in within the next 2-3 days.

Still to do before this can land:

• Tests for actually executing setup scripts
• Documentation
• Gate as an experimental feature
• Create a tracking issue covering open questions: Tracking issue for setup-scripts #978

[codecov]

Codecov Report

Merging #977 (81e167d) into main (c3b868b) will increase coverage by 1.04%.
The diff coverage is 84.04%.

❗ Current head 81e167d differs from pull request most recent head cebd379. Consider uploading reports for the commit cebd379 to get more accurate results

@@            Coverage Diff             @@
##             main     #977      +/-   ##
==========================================
+ Coverage   78.10%   79.15%   +1.04%     
==========================================
  Files          64       65       +1     
  Lines       14903    16543    +1640     
==========================================
+ Hits        11640    13094    +1454     
- Misses       3263     3449     +186     

Files	Coverage Δ
nextest-runner/src/config/retry_policy.rs	89.74% <100.00%> (+0.72%)	⬆️
nextest-runner/src/config/slow_timeout.rs	99.00% <100.00%> (+5.25%)	⬆️
nextest-runner/src/config/test_group.rs	88.18% <100.00%> (+0.15%)	⬆️
nextest-runner/src/config/test_threads.rs	91.81% <100.00%> (+0.47%)	⬆️
nextest-runner/src/config/threads_required.rs	96.93% <100.00%> (+0.19%)	⬆️
nextest-runner/src/config/tool_config.rs	94.90% <100.00%> (+0.11%)	⬆️
nextest-runner/src/list/test_list.rs	92.21% <100.00%> (+0.11%)	⬆️
cargo-nextest/src/dispatch.rs	80.16% <96.00%> (+0.22%)	⬆️
nextest-runner/src/reporter/aggregator.rs	39.73% <0.00%> (-0.14%)	⬇️
nextest-runner/src/config/overrides.rs	95.16% <97.67%> (+0.51%)	⬆️
... and 7 more

... and 2 files with indirect coverage changes

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