MINOR: mux-quic: implement rcv_buf

MINOR: mux-quic: implement rcv_buf

Implement the stream rcv_buf operation on QUIC mux.

A new buffer is stored in qcs structure named app_buf. This new buffer
will contains HTX and will be filled for example on H3 DATA frame
parsing.

The rcv_buf operation transfer as much as possible data from the HTX
from app_buf to the conn-stream buffer. This is mainly identical to
mux-h2. This is required to support HTTP POST data.