Combine RequestBody delegate, source state to fix hang

[adam-fowler]

The waitingForProduceMore continuations and the produceMore state have been combined into one state in the Delegate of RequestBody,

Fixes issue with hang where yield was told to stop producing and but then was told it could continue producing before the next call to yield.

An alternative solution, which is simpler, would be to wait on the continuation as soon as yield returns stopProducing but this means Source.yield() stalls on the yield that returns stop producing and not the next yield.

[Joannis]

LGTM, one performance remark here.

[codecov]

Codecov Report

Attention: Patch coverage is 87.34177% with 10 lines in your changes missing coverage. Please review.

Project coverage is 83.77%. Comparing base (b1bc64f) to head (cff29c0).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
Sources/HummingbirdCore/Request/RequestBody.swift	87.34%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #666      +/-   ##
==========================================
+ Coverage   83.69%   83.77%   +0.07%     
==========================================
  Files         116      116              
  Lines        7276     7339      +63     
==========================================
+ Hits         6090     6148      +58     
- Misses       1186     1191       +5     

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