Allow overriding resource mode in Jupyter Server Extension

[philippjfr]

Ensures that we can override the resource mode in the server extension by setting an environment variable.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 81.27%. Comparing base (f343b2b) to head (5dbd7b4).
Report is 1 commits behind head on main.

Files	Patch %	Lines
panel/io/jupyter_executor.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6965      +/-   ##
==========================================
- Coverage   81.71%   81.27%   -0.45%     
==========================================
  Files         326      326              
  Lines       48015    48017       +2     
==========================================
- Hits        39237    39024     -213     
- Misses       8778     8993     +215     

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