Generalize Tracer Framework and Add CMB Lensing Support

[vitenti]

Description

This PR generalizes Firecrown's tracer system to support multiple types of projected fields beyond galaxy redshift distributions, with CMB lensing as the first new implementation. The refactoring introduces a flexible ProjectedField protocol that enables the framework to work with different types of tracers while maintaining backwards compatibility.

Fixes #606, #607

Type of change

• New feature (non-breaking change which adds functionality)
• Refactoring

Checklist:

The following checklist will make sure that you are following the code style and
guidelines of the project as described in the
contributing page.

• I have run bash pre-commit-check and fixed any issues
• I have added tests that prove my fix is effective or that my feature works
• I have made corresponding changes to the documentation
• I have 100% test coverage for my changes (please check this after the CI system has verified the coverage)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (5d36572) to head (9718adb).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #616   +/-   ##
=======================================
  Coverage   100.0%   100.0%           
=======================================
  Files         151      151           
  Lines        9044     9076   +32     
  Branches     1033     1042    +9     
=======================================
+ Hits         9044     9076   +32     

Files with missing lines	Coverage Δ
firecrown/app/sacc/_utils.py	100.0% <100.0%> (ø)
firecrown/generators/_inferred_galaxy_zdist.py	100.0% <100.0%> (ø)
firecrown/likelihood/_cmb.py	100.0% <100.0%> (ø)
firecrown/likelihood/_two_point.py	100.0% <100.0%> (ø)
firecrown/likelihood/_weak_lensing.py	100.0% <100.0%> (ø)
firecrown/likelihood/number_counts/_factories.py	100.0% <100.0%> (ø)
firecrown/likelihood/number_counts/_source.py	100.0% <100.0%> (ø)
firecrown/metadata_functions/_combination_utils.py	100.0% <100.0%> (ø)
firecrown/metadata_functions/_extraction.py	100.0% <ø> (ø)
firecrown/metadata_functions/_matching.py	100.0% <100.0%> (ø)
... and 4 more

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