Allow async callbacks on FileDownload

[philippjfr]

Simple change to allow FileDownload.callback to accept an async callback.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (7eb9c71) 84.00% compared to head (86bb1d4) 84.00%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
panel/widgets/misc.py	94.44%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5878      +/-   ##
==========================================
- Coverage   84.00%   84.00%   -0.01%     
==========================================
  Files         290      290              
  Lines       42391    42409      +18     
==========================================
+ Hits        35609    35624      +15     
- Misses       6782     6785       +3     

Flag	Coverage Δ
ui-tests	40.88% <28.57%> (-0.02%)	⬇️
unitexamples-tests	72.36% <96.42%> (+<0.01%)	⬆️

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