Allow controlling DataFrame header and cell alignment

[philippjfr]

Fixes #6754

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 81.81%. Comparing base (a33d277) to head (9e13976).

Files	Patch %	Lines
panel/pane/markup.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7082      +/-   ##
==========================================
- Coverage   82.23%   81.81%   -0.42%     
==========================================
  Files         326      326              
  Lines       48531    48536       +5     
==========================================
- Hits        39909    39711     -198     
- Misses       8622     8825     +203     

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