Metrics with aggregators

[dafnapension]

Suggest a scheme of "aggregator" to be similarly employed by all three types of MetricsWithConfidenceInterval.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 90.78498% with 27 lines in your changes missing coverage. Please review.

Project coverage is 91.92%. Comparing base (a62c359) to head (b75c88e). Report is 23 commits behind head on main.

:exclamation: Current head b75c88e differs from pull request most recent head 132d114

Please upload reports for the commit 132d114 to get more accurate results.

Files	Patch %	Lines
src/unitxt/metrics.py	90.28%	27 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #845      +/-   ##
==========================================
+ Coverage   91.62%   91.92%   +0.29%     
==========================================
  Files         104      104              
  Lines       10996    10793     -203     
==========================================
- Hits        10075     9921     -154     
+ Misses        921      872      -49     

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[dafnapension]

replaced by #890