Add a way to set custom room version impls

[kegsay]

We have this lovely room version abstraction, but... no one can actually use it without getting it into this map. Let's fix that.

[codecov]

Codecov Report

Attention: Patch coverage is 52.94118% with 16 lines in your changes missing coverage. Please review.

Project coverage is 59.42%. Comparing base (c70b213) to head (38b41c3).

Files with missing lines	Patch %	Lines
eventauth.go	44.44%	15 Missing ⚠️
eventcontent.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #455      +/-   ##
==========================================
- Coverage   59.43%   59.42%   -0.01%     
==========================================
  Files          51       51              
  Lines        7257     7271      +14     
==========================================
+ Hits         4313     4321       +8     
- Misses       2544     2550       +6     
  Partials      400      400              

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[devonh]

Other than a potential test addition, this looks perfectly reasonable to me.