Unified write data API

[azkrishpy]

Issue #, if available:

Description of changes:
A new protocol-agnostic aws_http_stream_write_data() API that lets users write request body data the same way regardless of whether the connection is HTTP/1.1 or HTTP/2.

High level details of changes:
Public API:

• New aws_http_stream_write_data() function that works for both H1 and H2
• New use_manual_data_writes flag in request options (unified replacement for the H2-only http2_use_manual_data_writes)
• Both old APIs (aws_http1_stream_write_chunk, aws_http2_stream_write_data) still work for backward compatibility

H1 Content-Length path:

• User sets Content-Length header + use_manual_data_writes = true, no body stream
• Each write_data call creates an aws_h1_data_write that gets queued and processed by the encoder
• New encoder states (DATA_WRITE_NEXT, DATA_WRITE_BODY) handle reading from queued writes and tracking progress against Content-Length
• Encoder validates total bytes match Content-Length when end_stream = true

H1 Chunked path:

• User sets Transfer-Encoding: chunked header + use_manual_data_writes = true
• write_data internally converts to write_chunk calls (same underlying mechanism as before)
• User sends a zero-length write_data with end_stream = true to terminate (consistent with write_chunk behavior)

H2 path:

• use_manual_data_writes now works for H2 (alongside the existing http2_use_manual_data_writes)
• Internally routes through the same vtable->write_data as before — H2 implementation unchanged

Vtable unification:

• Single write_data vtable entry replaces the old http2_write_data
• H1 stream implements it (new), H2 stream implements it (renamed from existing)
• aws_http2_stream_write_data() casts options and calls the same vtable entry

Elasticurl example:

• Added --manual-write interactive mode demonstrating the API

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

❌ Patch coverage is 85.33333% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.75%. Comparing base (a9745ea) to head (f68f849).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
source/h1_stream.c	84.41%	12 Missing ⚠️
source/h1_encoder.c	83.33%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #552      +/-   ##
==========================================
+ Coverage   79.72%   79.75%   +0.02%     
==========================================
  Files          28       28              
  Lines       11894    12015     +121     
==========================================
+ Hits         9483     9583     +100     
- Misses       2411     2432      +21     

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[azkrishpy]

should we switch to having encoder options instead of adding more parameters?