Add filter_by kwarg to serialize for ChatFeed

[ahuang11]

Closes #6048

If I'm fine-tuning and I only want some responses, I want to make it easier to only select the messages with the desired reactions

[codecov]

Codecov Report

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

Comparison is base (5b89692) 84.66% compared to head (d6574c9) 84.66%.

Files	Patch %	Lines
panel/chat/interface.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6090      +/-   ##
==========================================
- Coverage   84.66%   84.66%   -0.01%     
==========================================
  Files         296      296              
  Lines       44057    44068      +11     
==========================================
+ Hits        37302    37310       +8     
- Misses       6755     6758       +3     

Flag	Coverage Δ
ui-tests	40.62% <11.76%> (-0.01%)	⬇️
unitexamples-tests	72.73% <94.11%> (-0.01%)	⬇️

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