Add DeprecatedDict mapping

[fred3m]

DeprecatedDict is a dict that warns the first time a deprecated key is read or written through targeted single-key access, while leaving bulk operations (copy, iterate, serialize, merge) silent. It eases migrating a field out of a mapping and onto a typed attribute while keeping the old key readable during a deprecation period.

[codecov]

Codecov Report

❌ Patch coverage is 99.20635% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.73%. Comparing base (e52f197) to head (df66058).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
python/lsst/utils/deprecated.py	98.11%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #243      +/-   ##
==========================================
+ Coverage   94.59%   94.73%   +0.14%     
==========================================
  Files          51       51              
  Lines        3883     4007     +124     
==========================================
+ Hits         3673     3796     +123     
- Misses        210      211       +1     

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[timj]

@fred3m I'm confused as to how you deleted the PR template for this repo that was asking you to make sure you had included a news fragment for the release notes...

[fred3m]

Sorry, I didn't think that this was a release notes worthy addition.

[timj]

For utils a whole new API is definitely release notes worthy 😄

[timj]

I'll try to remember to add a note about it next week when I make the v30 release.