HTTP/1: Support streaming requests of unknown length

[graebm]

Issue:

aws-sdk-cpp needs to send streaming requests of unknown content length. This isn't currently supported in our HTTP/1 client.

Background:

The HTTP/1 client already supported these use cases:

• Send streaming request body, but declare Content-Length up front
• Send chunked request body, but user must declare length of each chunk via write_chunk() API.

But it wasn't possible to send a stream of unknown length without doing extra copies (read to intermediate buffer, then send that buffer via write_chunk() API).

Description of changes:

Support input streams of unknown length via chunked encoding.

When reading from the stream, leave a bit of space at the start and end of the aws_io_message buffer for the chunk prefix and suffix. Then go back and write in the prefix and suffix afterwards. No unnecessary copies are involved.

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

Attention: Patch coverage is 87.50000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 79.32%. Comparing base (60c43f8) to head (d1b0d70).

Files with missing lines	Patch %	Lines
source/h1_encoder.c	87.50%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #506      +/-   ##
==========================================
- Coverage   79.41%   79.32%   -0.10%     
==========================================
  Files          27       27              
  Lines       11621    11666      +45     
==========================================
+ Hits         9229     9254      +25     
- Misses       2392     2412      +20     

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