Tests for async streaming (seems to work!)

[graebm]

• Add basic tests for async streaming of the request body.
• When async streaming, don't let more than 1 part at a time be in the "prepare" stage.
  • since aws_async_input_stream doesn't allow overlapping read() calls.
• Stop wrapping synchronous streams in an async stream.
  • Now that we treat async streaming slightly different than synchronous, it's simpler to just have them be separate things.

Future work:

• more tests
• benchmarking / optimization

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Merging #301 (08db35f) into main (931d258) will decrease coverage by 0.09%.
The diff coverage is 93.75%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #301      +/-   ##
==========================================
- Coverage   85.97%   85.89%   -0.09%     
==========================================
  Files          17       17              
  Lines        4921     4928       +7     
==========================================
+ Hits         4231     4233       +2     
- Misses        690      695       +5     

Impacted Files	Coverage Δ
source/s3_meta_request.c	92.71% <90.90%> (-0.73%)	⬇️
source/s3_request_messages.c	70.85% <95.00%> (-0.14%)	⬇️
source/s3_auto_ranged_put.c	93.25% <100.00%> (+0.15%)	⬆️

... and 2 files with indirect coverage changes