feat(similarity): Add read only flag to NN endpoint

[jangjodi]

Add read_only flag to get_nearest_neighbor endpoint

[JoshFerge]

should we make this not a parameter that's passed in, but just default? any situation where we'd want to run embeddings if the endpoint is hit?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.91%. Comparing base (20e4b53) to head (3fdabb3).
Report is 12 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #72021       +/-   ##
===========================================
+ Coverage   56.88%   77.91%   +21.02%     
===========================================
  Files        6562     6572       +10     
  Lines      292299   292722      +423     
  Branches    50462    50533       +71     
===========================================
+ Hits       166273   228061    +61788     
+ Misses     121283    58408    -62875     
- Partials     4743     6253     +1510     

Files	Coverage Δ
...y/api/endpoints/group_similar_issues_embeddings.py	100.00% <ø> (+47.36%)	⬆️
src/sentry/seer/similarity/types.py	100.00% <100.00%> (+17.30%)	⬆️

... and 1969 files with indirect coverage changes