Tabulator aggregators: allow nested dict, fix data aggregation

[thuydotm]

Fixes #7281 and partially #7449

TODO:

• add tests
• add docs for nested aggs

TODO LATER in other PRs:

• validate input aggs?
• for columns with no agg method, set NaNs to them? Needs a bit more experiment to see what we should do
• display the indexes properly
• scope out the data types to support. Handle String, Datetime, and Boolean data types?

[codecov]

Codecov Report

Attention: Patch coverage is 42.85714% with 56 lines in your changes missing coverage. Please review.

Project coverage is 82.31%. Comparing base (8dd362f) to head (4324b3b).
Report is 36 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/tests/ui/widgets/test_tabulator.py	36.36%	56 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7450      +/-   ##
==========================================
+ Coverage   82.08%   82.31%   +0.23%     
==========================================
  Files         337      339       +2     
  Lines       50680    51090     +410     
==========================================
+ Hits        41599    42054     +455     
+ Misses       9081     9036      -45     

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[philippjfr]

Great! Some tests would be good.

[maximlt]

@thuydotm I just pushed some minor docs changes, I think we're good on this front! I have left a few questions and suggestions for the code and tests. Pretty hard to review this as there are so many other issues with the aggregators 🙃 This first PR is almost done, we'll get there!

[maximlt]

LGTM!