Add event size checks similar to Synapse

[S7evinK]

Companion to matrix-org/gomatrixserverlib#400
This tries to mimic the logic found in Synapse, as dropping events can break rooms (and we may end up in endless loops..)

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.01 🎉

Comparison is base (cc9b695) 65.25% compared to head (ff05a40) 65.26%.

❗ Current head ff05a40 differs from pull request most recent head 772228e. Consider uploading reports for the commit 772228e to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3140      +/-   ##
==========================================
+ Coverage   65.25%   65.26%   +0.01%     
==========================================
  Files         504      504              
  Lines       56212    56113      -99     
==========================================
- Hits        36679    36624      -55     
+ Misses      15713    15668      -45     
- Partials     3820     3821       +1     

Flag	Coverage Δ
unittests	49.69% <0.00%> (-0.07%)	⬇️

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Impacted Files	Coverage Δ
roomserver/internal/input/input_events.go	56.06% <0.00%> (-2.68%)	⬇️
roomserver/internal/input/input_missing.go	62.67% <0.00%> (-5.51%)	⬇️

... and 31 files with indirect coverage changes

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