DM-40198 Allow extra local variables when loading configs

[natelust]

Allow a dictionary of variables to be used in local scope when configs are loaded from a stream or string.

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.04% 🎉

Comparison is base (733a212) 85.33% compared to head (07727ac) 85.37%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #104      +/-   ##
==========================================
+ Coverage   85.33%   85.37%   +0.04%     
==========================================
  Files          46       46              
  Lines        3621     3631      +10     
==========================================
+ Hits         3090     3100      +10     
  Misses        531      531              

Files Changed	Coverage Δ
python/lsst/pex/config/config.py	90.69% <100.00%> (+0.08%)	⬆️
tests/test_Config.py	95.89% <100.00%> (+0.06%)	⬆️

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[TallJimbo]

I've heard that we can and should just remove the --- in .github/workflows/rebase_checker.yaml that yields the warning at the bottom of the PR when we see it.