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 (055b907) to head (a262b38).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master     #616    +/-   ##
========================================
  Coverage   100.0%   100.0%            
========================================
  Files         150      151     +1     
  Lines        8721     8977   +256     
  Branches     1038     1063    +25     
========================================
+ Hits         8721     8977   +256     

Files with missing lines	Coverage Δ
firecrown/app/analysis/_analysis_builder.py	100.0% <100.0%> (ø)
firecrown/app/analysis/_cobaya.py	100.0% <100.0%> (ø)
firecrown/app/analysis/_config_generator.py	100.0% <ø> (ø)
firecrown/app/analysis/_cosmosis.py	100.0% <100.0%> (ø)
firecrown/app/analysis/_numcosmo.py	100.0% <ø> (ø)
firecrown/app/analysis/_types.py	100.0% <100.0%> (ø)
firecrown/app/examples/__init__.py	100.0% <100.0%> (ø)
firecrown/app/examples/_cmb_cross.py	100.0% <100.0%> (ø)
firecrown/app/sacc/_utils.py	100.0% <100.0%> (ø)
firecrown/app/sacc/_view.py	100.0% <100.0%> (ø)
... and 18 more

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[marcpaterno]

The only concern I have is with backward compatibility. This PR takes care to support existing code that imports the old names. But we may break user code that relies on existing serialized YAML that contains old class names. Do we need to support reading old serialized data?

I would like to be able to say that we explicitly do not, because I think trying to do so will substantially complicate the code for what I suspect is little value. But perhaps we need to get agreement in a Firecrown/CCL meeting that the value is small.