Add support for decorating async functions with pn.io.cache

[philippjfr]

Fixes #4236

[codecov]

Codecov Report

Merging #5649 (da3d86f) into main (a6dcce9) will decrease coverage by 59.50%.
Report is 4 commits behind head on main.
The diff coverage is 18.91%.

@@             Coverage Diff             @@
##             main    #5649       +/-   ##
===========================================
- Coverage   83.57%   24.08%   -59.50%     
===========================================
  Files         280      280               
  Lines       41599    41626       +27     
===========================================
- Hits        34766    10025    -24741     
- Misses       6833    31601    +24768     

Flag	Coverage Δ
ui-tests	?
unitexamples-tests	24.08% <18.91%> (-49.53%)	⬇️

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Files	Coverage Δ
panel/tests/io/test_cache.py	25.76% <20.00%> (-71.50%)	⬇️
panel/io/cache.py	28.18% <18.51%> (-56.12%)	⬇️

... and 221 files with indirect coverage changes

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