web: support streaming request bodies

[yasunariw]

Add Web.http_request_lwt_streaming_body' for buffered streaming uploads with a known content length, using Content-Length rather than chunked transfer encoding. Curl pauses when the buffer is empty and resumes as data arrives.

The function accepts a streaming_body constructed with the content type, content length, and read/close callbacks. Its buffer_size controls read-ahead.

It parallels http_request_lwt' but is separate because asynchronous bodies require pause/resume and producer cleanup.

*This raises the minimal ocurl version to 0.9.2, which introduces Curl.set_readfunction2 exposing pause/abort return values.

How to test

Includes a loopback test that uploads and verifies a generated 512-1024 KiB body.

dune runtest