Add MultiAxisTap stream

[philippjfr]

Retrofitting the existing Tap stream with support for multiple axis was too awkward. This PR instead implements a MultiAxisTap stream which provides xs and ys parameters giving you the tap locations indexed by the name of the dimension.

[codecov]

Codecov Report

Attention: Patch coverage is 81.57895% with 14 lines in your changes missing coverage. Please review.

Project coverage is 88.49%. Comparing base (099d3de) to head (21a12e0).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
holoviews/plotting/bokeh/callbacks.py	76.47%	8 Missing ⚠️
holoviews/tests/ui/bokeh/test_callback.py	80.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6374      +/-   ##
==========================================
- Coverage   88.50%   88.49%   -0.01%     
==========================================
  Files         323      323              
  Lines       68194    68265      +71     
==========================================
+ Hits        60355    60412      +57     
- Misses       7839     7853      +14     

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