feat(llc, samples): basic stories implementation

[renefloor]

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Closes FLU-297
Closes FLU-298

CLA

• I have signed the Stream CLA (required).
• The code changes follow best practices
• Code changes are tested (add some information if not applicable)

Description of the pull request

This adds stories to the sample app and some improvements in feed updates for aggregated activities.

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.81%. Comparing base (11b075c) to head (ad8bbc9).

Files with missing lines	Patch %	Lines
...ackages/stream_feeds/lib/src/state/feed_state.dart	84.61%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #50      +/-   ##
==========================================
+ Coverage   40.86%   42.81%   +1.95%     
==========================================
  Files         120      120              
  Lines        3617     3646      +29     
==========================================
+ Hits         1478     1561      +83     
+ Misses       2139     2085      -54     

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