feat: Add allowlist_file option to yaml migrators

[zklaus]

Description:

This PR adds a new allowlist_file option to the yaml specification for migrators. This can be used to limit the packages affected by a migrator with a text file listing desired packages much like we already use for arch migrations.

This came out of discussions around the currently paused Python free-threaded migrator to give us the option of approaching that migration carefully, but may find applications beyond that.

Checklist:

• Pydantic model updated or no update needed

Cross-refs, links to issues, etc:

[isuruf]

Please add to docs at

cf-scripts/README.md

Line 151 in 2fb5a12

[beckermr]

We need to add a test of dumping and loading migrators as JSON with this new option being used.

[codecov]

Codecov Report

❌ Patch coverage is 93.10345% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.65%. Comparing base (2fb5a12) to head (8b227bd).

Files with missing lines	Patch %	Lines
conda_forge_tick/migrators/migration_yaml.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4858      +/-   ##
==========================================
- Coverage   80.66%   80.65%   -0.02%     
==========================================
  Files         143      143              
  Lines       16492    16492              
==========================================
- Hits        13304    13302       -2     
- Misses       3188     3190       +2     

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