Add activity indicator to ChatMessage

[ahuang11]

Pretty happy with this implementation; shows dot while streaming:

Screen.Recording.2024-01-04.at.11.00.14.AM.mov

(The extraneous padding/shifting of the message is fixed in the later commits)

[codecov]

Codecov Report

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

Comparison is base (46e0639) 84.69% compared to head (4ef6cfe) 84.60%.
Report is 6 commits behind head on main.

❗ Current head 4ef6cfe differs from pull request most recent head c08144c. Consider uploading reports for the commit c08144c to get more accurate results

Files	Patch %	Lines
panel/chat/feed.py	94.11%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6153      +/-   ##
==========================================
- Coverage   84.69%   84.60%   -0.09%     
==========================================
  Files         296      296              
  Lines       44184    44196      +12     
==========================================
- Hits        37423    37394      -29     
- Misses       6761     6802      +41     

Flag	Coverage Δ
ui-tests	40.72% <8.69%> (-0.02%)	⬇️
unitexamples-tests	72.53% <95.65%> (-0.09%)	⬇️

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