Add sticky event parsing

[turt2live]

MSC: matrix-org/matrix-spec-proposals#4354

This is for policyserv so we can block sticky events in our public rooms (they aren't needed there)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 54.07%. Comparing base (6697d93) to head (9bcabf1).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #462      +/-   ##
==========================================
+ Coverage   53.43%   54.07%   +0.64%     
==========================================
  Files          52       52              
  Lines        7842     5993    -1849     
==========================================
- Hits         4190     3241     -949     
+ Misses       3247     2342     -905     
- Partials      405      410       +5     

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[kegsay]

Otherwise LGTM, thanks!