BUG/MINOR: quic: support FIN on Rx-buffered STREAM frames

BUG/MINOR: quic: support FIN on Rx-buffered STREAM frames

FIN flag on a STREAM frame was not detected if the frame was previously
buffered on qcs.rx.frms before being handled.

To fix this, copy the fin field from the quic_stream instance to
quic_rx_strm_frm. This is required to properly notify the FIN flag on
qc_treat_rx_strm_frms for the MUX layer.

Without this fix, the request channel might be left opened after the
last STREAM frame reception if there is out-of-order frames on the Rx
path.