[CHORE] Add endpoints to simulate rate-limiting on AWS S3 buckets

[jaychia]

Summary

Adds FastAPI endpoints for integration tests, simulating rate-limiting on AWS S3 buckets.

Drive-by: refactors our code to use FastAPI sub-apps instead of Routers. This is needed to enable better control around rate limiting per-"app", where each app corresponds to an S3 bucket with certain characteristics.

Closes: #1179

More work needs to be done here once we support customizable retry policies, so that we can verify that these retry strategies work.

[codecov]

Codecov Report

Merging #1220 (f2dfbd4) into main (e95acae) will decrease coverage by 0.02%.
Report is 4 commits behind head on main.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1220      +/-   ##
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- Coverage   88.43%   88.41%   -0.02%     
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  Files          55       55              
  Lines        5620     5620              
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- Hits         4970     4969       -1     
- Misses        650      651       +1     

see 1 file with indirect coverage changes