[nextest-runner] generate and add schema for user config

[sunshowers]

Similar to the recently-added schema for repository config.

[codecov]

Codecov Report

❌ Patch coverage is 4.76190% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.52%. Comparing base (a59d170) to head (672f7dd).

Files with missing lines	Patch %	Lines
cargo-nextest/src/dispatch/utility/self_cmd.rs	0.00%	16 Missing ⚠️
cargo-nextest/src/errors.rs	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3351      +/-   ##
==========================================
- Coverage   85.55%   85.52%   -0.04%     
==========================================
  Files         160      160              
  Lines       47942    47954      +12     
==========================================
- Hits        41018    41012       -6     
- Misses       6924     6942      +18     

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[trunk-io]

     

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