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

[codecov]

Codecov Report

Attention: Patch coverage is 38.98305% with 72 lines in your changes missing coverage. Please review.

Project coverage is 82.27%. Comparing base (8dd362f) to head (d08363e).
Report is 27 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/tests/ui/widgets/test_tabulator.py	38.46%	72 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7450      +/-   ##
==========================================
+ Coverage   82.08%   82.27%   +0.19%     
==========================================
  Files         337      339       +2     
  Lines       50680    51112     +432     
==========================================
+ Hits        41599    42052     +453     
+ Misses       9081     9060      -21     

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[philippjfr]

Great! Some tests would be good.