bench(event cache): add a benchmark for measuring how long it takes to handle a sync update

[bnjbvr]

Each sync update contains N rooms (for N in 1, 10, 100), each gets 1000 new room message (text) events. We might want to do something more representative, as we've done in other benchmarks (throw a few aggregated events in the mix), but for now that'll be sufficient, and it's already teaching us things, notably that the supposed performance improvement in #5426 isn't the obvious improvement I thought it would be. More investigation is needed there, so in the meantime I'll just add a comment that it's not as good as it was supposed to be.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 88.86%. Comparing base (bcb4ab4) to head (9a71765).
Report is 6 commits behind head on main.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...es/matrix-sdk-base/src/event_cache/store/traits.rs	0.00%	3 Missing ⚠️
crates/matrix-sdk/src/event_cache/mod.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5435   +/-   ##
=======================================
  Coverage   88.86%   88.86%           
=======================================
  Files         333      333           
  Lines       91374    91380    +6     
  Branches    91374    91380    +6     
=======================================
+ Hits        81199    81205    +6     
  Misses       6351     6351           
  Partials     3824     3824           

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