Add predict_conditional_average_outcomes method to R-Learner

[kklein]

Checklist

• Added a CHANGELOG.rst entry

[codecov]

Codecov Report

Attention: Patch coverage is 94.73684% with 1 line in your changes missing coverage. Please review.

Project coverage is 94.42%. Comparing base (4409cc5) to head (53edb1d).
Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
metalearners/metalearner.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #78   +/-   ##
=======================================
  Coverage   94.41%   94.42%           
=======================================
  Files          15       15           
  Lines        1774     1793   +19     
=======================================
+ Hits         1675     1693   +18     
- Misses         99      100    +1     

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