feat: support arbitrary field dependents

[tlambert03]

This is a proof of principle PR that would support the feature that @sjdemartini is requesting in https://github.com/pyapp-kit/psygnal/issues/337. namely:

• computed fields can gain events
• computed fields (really, currently any field) can list non-field attribute names as their dependents for checking event emission.

@Czaki, I would appreciate your eyes on this.

1. I just "made it work" ... so there's likely some minor refactoring that should be done here to polish it (particularly in the _check_if_values_changed_and_emit_if_needed area that you had previously worked on)
2. Feel free to comment in general on the feature request. I think @sjdemartini motivated it quite nicely his comment, https://github.com/pyapp-kit/psygnal/issues/337#issuecomment-2472157973 ... so please read that for context; and for a quick use case example, see the new test I added in test_private_field_dependents

closes #337

[tlambert03]

tests are failing during building of mypyc, due to https://github.com/pydantic/pydantic/issues/10907

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #340 will not alter performance

_{Comparing tlambert03:computed-fields (9e4f833) with main (185fc4e)}

Summary

✅ 67 untouched benchmarks

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 99.86%. Comparing base (185fc4e) to head (9e4f833). :warning: Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #340      +/-   ##
==========================================
- Coverage   99.91%   99.86%   -0.05%     
==========================================
  Files          22       22              
  Lines        2230     2248      +18     
==========================================
+ Hits         2228     2245      +17     
- Misses          2        3       +1     

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