Add streamed response callback with chunked transfer support

[Kirpatik]

Summary

This PR adds explicit streamed response sending via callback, including support for unknown total size responses.

Added API

• Known-length streaming:
  • set_streamed_body(std::function<size_t(void*, size_t)> reader, size_t content_length, std::string content_type = "", size_t chunk_size = 16 * 1024)
• Unknown-length streaming:
  • set_streamed_body(std::function<size_t(void*, size_t)> reader, std::string content_type = "", size_t chunk_size = 16 * 1024)

Callback contract:

• Crow provides (buffer, max_size)
• callback writes up to max_size bytes
• callback returns written size
• callback returns 0 to indicate end-of-stream

Behavior

• HTTP/1.1+ unknown-size responses are sent using Transfer-Encoding: chunked.
• HTTP/1.0 falls back to connection-close delimited streaming (no chunked encoding).
• chunk_size is configurable per response; default remains 16KB.
• HEAD with known-length streamed responses sends headers only and preserves Content-Length.

Implementation Notes

• Streaming logic is handled in Connection::do_write_streamed().
• Response metadata now stores stream mode and chunk size.
• Existing non-streaming behavior is unchanged.

Tests

Added/updated tests:

• streamed_response
• streamed_response_unknown_size_chunked
• streamed_response_unknown_size_http10_fallback
• streamed_response_head_known_length_no_body

[gittiver]

Sounds great. :)

[gittiver]

The current implementation would block one working thread per running client request, right?

[Kirpatik]

The current implementation would block one working thread per running client request, right?

Yes.

do_write_streamed() is called synchronously from complete_request() and performs blocking asio::write calls in a loop, so one worker thread stays occupied by that request until the stream finishes.