MINOR: mux-quic: add an sctl() callback to the application ops

MINOR: mux-quic: add an sctl() callback to the application ops

The QUIC mux will need to pass some stream controls to the application
layer, which holds information that the mux doesn't know about (e.g. the
HTTP hardening results of the H3 stream). This adds a new sctl() callback
to qcc_app_ops, which takes the qcs, the mux_sctl_type and an output
pointer, and works like mux->sctl(), returning -1 when not supported.

For now, h3_sctl() doesn't implement anything and the mux doesn't call it
yet.