MINOR: mux-quic: add a app-layer context in qcs

MINOR: mux-quic: add a app-layer context in qcs

Define 2 new callback for qcc_app_ops : attach and detach. They are
called when a qcs instance is respectively allocated and freed. If
implemented, they can allocate a custom context stored in the new
abstract field ctx of qcs.

For now, h3 and hq-interop does not use these new callbacks. They will
be soon implemented by the h3 layer to allocate a context used for
stateful demuxing.

This change is required to support the demuxing of H3 frames bigger than
a buffer.