Add playback OpenAPI definition

[bouk]

Makes it easy to generate a client for it

Fixes #5545

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.86%. Comparing base (9f94bd1) to head (916c93c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5546      +/-   ##
==========================================
+ Coverage   60.84%   60.86%   +0.01%     
==========================================
  Files         200      200              
  Lines       16865    16865              
==========================================
+ Hits        10262    10265       +3     
+ Misses       5732     5728       -4     
- Partials      871      872       +1     

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