[CHORE] Add s3 fixtures for retrying logic

[jaychia]

Adds FastAPI fixtures for testing s3 error handling and retry logic

The current server serves two buckets:

1. s3://head-retries-bucket/{status_code}/{num_errors}/{item_id}: for a given item_id, will throw the status code num_errors times on HEAD requests before giving a successful response
2. s3://get-retries-bucket/{status_code}/{num_errors}/{item_id}: for a given item_id and byte-range, will throw the status code num_errors times on ranged GET requests before giving a successful response

[codecov]

Codecov Report

Merging #1206 (566a058) into main (bc11e57) will increase coverage by 0.01%.
Report is 10 commits behind head on main.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1206      +/-   ##
==========================================
+ Coverage   88.40%   88.41%   +0.01%     
==========================================
  Files          55       55              
  Lines        5606     5620      +14     
==========================================
+ Hits         4956     4969      +13     
- Misses        650      651       +1     

see 2 files with indirect coverage changes