Async streaming - WIP

[graebm]

Work in Progress support for async streaming of the request body. We've tested that this doesn't break the existing synchronous streaming. But actual async streaming hasn't been tested yet, there are sure to be bugs. Committing to main now, before it's 100% complete, so it's simpler to continue working on this in parallel.

These changes have already been reviewed in the following incremental PRs:

• Async streaming 1st pass #290
• Async stream test #293
• Move total_num_parts to non synced data. #295
• Adapt to aws_future and aws_async_input_stream API changes #296
• Address feedback from initial PR #298

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 #299 (3194636) into main (21b0a10) will increase coverage by 0.04%.
The diff coverage is 94.50%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #299      +/-   ##
==========================================
+ Coverage   85.77%   85.82%   +0.04%     
==========================================
  Files          17       17              
  Lines        4717     4910     +193     
==========================================
+ Hits         4046     4214     +168     
- Misses        671      696      +25     

Impacted Files	Coverage Δ
source/s3.c	96.15% <ø> (ø)
source/s3_client.c	87.91% <66.66%> (-0.32%)	⬇️
source/s3_copy_object.c	36.94% <80.00%> (+1.65%)	⬆️
source/s3_auto_ranged_put.c	93.09% <94.89%> (+0.41%)	⬆️
source/s3_request_messages.c	70.99% <95.45%> (+0.13%)	⬆️
source/s3_default_meta_request.c	94.24% <97.43%> (+1.10%)	⬆️
source/s3_auto_ranged_get.c	98.18% <100.00%> (-0.35%)	⬇️
source/s3_meta_request.c	92.81% <100.00%> (-1.75%)	⬇️