WIP: h3: add body_capacity() helper

[LPardue]

Previously, quiche kept all details about the relationship between stream capacity and writable DATA internal. This could lead to awkward situations where quiche would report a stream as writable, quiche::conn::stream_capacity() could report a non-zero value, but send_body() might not send anything due to DATA frame overheads.

This change introduces the quiche::h3::conn::body_capacity() helper function to help applications determine the amount of usable capacity.