Introduce output_stream writing shugar

[xemul]

When putting data into output_stream() a common pattern exists. Data is written into stream, then flushed and the end and the stream is closed. Any exception that may pop up in the middle shouldn't leave stream not closed, so in the code it looks smth like

    try {
        for (...) {
           co_await out.write()
        }
        co_await out.flush()
    } catch () {
        ex = current_exception()
    }
    co_await out.close()
    if (ex) {
        throw ex
    }

This is very typical to http client and handlers code that use request::write_bod() and response::write_body() overloads that call user-provided lambda giving it chunked or content-length output_stream. The caller then implements the paterns described above on its own.

This PR suggest a pair of helpers -- one is the util::write_to_stream() function that looks literally like above; the other one is request and response .write_body() overloads that accept a callback doing only the out.write() (for the loop) part of the above, flushing and closing the stream on its own.

[xemul]

upd:

• new helper name is write_to_stream_and_close
• added documentation to that one