MEDIUM: mux-h2: support passing H2 DATA frames to HTX blocks

MEDIUM: mux-h2: support passing H2 DATA frames to HTX blocks

This is used for uploads, we can now convert H2 DATA frames to HTX
DATA blocks. It's uncertain whether it's better to reuse the same
function or to split it in two at this point. For now the same
function was added with some paths specific to HTX. In this mode
we loop back to the same or next frame in order to try to complete
DATA blocks.