Add mean dispatch for pymc distributions

[lucianopaz]

Description

This PR provides a single dispatch protocol that computes the expected value of most pymc distributions.

Related Issue

• Closes #
• Related to #

Checklist

• Checked that the pre-commit linting/style checks pass
• Included tests that prove the fix is effective or that the new feature works
• Added necessary documentation (docstrings and/or example notebooks)
• If you are a pro: each commit corresponds to a relevant logical change

Type of change

• New feature / enhancement
• Bug fix
• Documentation
• Maintenance
• Other (please specify):

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📚 Documentation preview 📚: https://pymc--7530.org.readthedocs.build/en/7530/

[lucianopaz]

@ricardoV94, I finally decided to put some time and port this stuff into pymc

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.81%. Comparing base (938aff4) to head (1c71deb).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7530      +/-   ##
==========================================
+ Coverage   92.69%   92.81%   +0.11%     
==========================================
  Files         104      106       +2     
  Lines       17409    17644     +235     
==========================================
+ Hits        16138    16376     +238     
+ Misses       1271     1268       -3     

Files with missing lines	Coverage Δ
pymc/distributions/moments/__init__.py	100.00% <100.00%> (ø)
pymc/distributions/moments/means.py	100.00% <100.00%> (ø)
pymc/exceptions.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

[ricardoV94]

Looks great. Small change requests commented above