Add overloads for working with capy/corosio #280

When trying to write a simple server, I noticed that when writing to a corosio tcp_socket creating the const buffer for the write is rather ugly. Since these libraries are already so tied together methods/overloads for working with capy/corosio would be nice. For example this:

        http::response res;
        res.set_status(http::status::ok);
        auto [ec, n] = co_await m_socket.write_some(
            capy::const_buffer{res.buffer().data(), res.buffer().size()});

Unless I missed some very obvious method, there is no nice way to pass an http::request to write_some. Just .buffer() doesn't work as write_some doesn't accept a string_view.