feat: AsyncMultipartStreamParser

[defnull]

An async-aware wrapper for PushMultipartParser that reads bytes from an awaitable read function on demand and returns AsyncMultipartPart instances that offer convenient async read methods for their payload.

This is still a stream parser, which means that parts are not buffered to disk and can only be read once, and only in the order they appear in the multipart stream. It is still WAY more convenient than using PushMultipartParser directly.

Example usage:

import multipart

async def handle_request(headers: dict[str, str], body_reader: multipart.t_AsyncReader):
    ctype, options = multipart.parse_options_header(headers["content-type"])
    assert ctype == "multipart/form-data"
    parser = multipart.AsyncMultipartStreamParser(
        multipart.PushMultipartParser(boundary=options["boundary"]), body_reader
    )

    async for part in parser:
        print(f"Found: {part.name} ({part.filename or '-'})")
        async for chunk in part.iter_chunks():
            print(f"[{len(chunk)} bytes]")
        print(f"Total size: {part.size}")

Work in Progress

• Think about a nice(r) API for async applications
• Implement stuff
• Get test coverage back to 100%
• Improve documentation
• Benchmark this properly