Add getting/deleting single event report

[S7evinK]

Based on #3342

Adds GET /_synapse/admin/v1/event_reports/{reportID} and DELETE /_synapse/admin/v1/event_reports/{reportID}

[codecov]

Codecov Report

Attention: Patch coverage is 76.92308% with 30 lines in your changes are missing coverage. Please review.

Project coverage is 67.40%. Comparing base (1bdf0cc) to head (899b622).
Report is 2 commits behind head on main.

Files	Patch %	Lines
roomserver/storage/shared/storage.go	58.13%	10 Missing and 8 partials ⚠️
clientapi/routing/admin.go	71.42%	6 Missing and 2 partials ⚠️
clientapi/routing/routing.go	69.23%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3344      +/-   ##
==========================================
- Coverage   67.41%   67.40%   -0.01%     
==========================================
  Files         521      521              
  Lines       47027    47348     +321     
==========================================
+ Hits        31702    31915     +213     
- Misses      11362    11429      +67     
- Partials     3963     4004      +41     

Flag	Coverage Δ
unittests	52.74% <76.92%> (+0.19%)	⬆️

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