MINOR: h3/hq-interop: restore function for standalone FIN receive

MINOR: h3/hq-interop: restore function for standalone FIN receive

Previously, a function qcs_http_handle_standalone_fin() was implemented
to handle a received standalone FIN, bypassing app_ops layer decoding.
However, this was removed as app_ops layer interaction is necessary. For
example, HTTP/3 checks that FIN is never sent on the control uni stream.

This patch reintroduces qcs_http_handle_standalone_fin(), albeit in a
slightly diminished version. Most importantly, it is now the
responsibility of the app_ops layer itself to use it, to avoid the
shortcoming described above.

The main objective of this patch is to be able to support standalone FIN
in HTTP/0.9 layer. This is easily done via the reintroduction of
qcs_http_handle_standalone_fin() usage. This will be useful to perform
testing, as standalone FIN is a corner case which can easily be broken.