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.
diff --git a/src/mux_quic.c b/src/mux_quic.c
index 17adf46..5c5ba2c 100644
--- a/src/mux_quic.c
+++ b/src/mux_quic.c
@@ -116,6 +116,7 @@
 	qcs->qcc = qcc;
 	qcs->cs = NULL;
 	qcs->flags = QC_SF_NONE;
+	qcs->ctx = NULL;
 
 	/* allocate transport layer stream descriptor
 	 *
@@ -126,6 +127,10 @@
 	if (!qcs->stream)
 		goto err;
 
+	if (qcc->app_ops->attach) {
+		if (qcc->app_ops->attach(qcs))
+			goto err;
+	}
 
 	qcs->endp = cs_endpoint_new();
 	if (!qcs->endp) {
@@ -169,6 +174,9 @@
 	return qcs;
 
  err:
+	if (qcs->ctx && qcc->app_ops->detach)
+		qcc->app_ops->detach(qcs);
+
 	if (qcs->stream)
 		qc_stream_desc_release(qcs->stream);
 
@@ -188,6 +196,9 @@
 	BUG_ON(!qcs->qcc->strms[qcs_id_type(qcs->id)].nb_streams);
 	--qcs->qcc->strms[qcs_id_type(qcs->id)].nb_streams;
 
+	if (qcs->ctx && qcs->qcc->app_ops->detach)
+		qcs->qcc->app_ops->detach(qcs);
+
 	qc_stream_desc_release(qcs->stream);
 
 	BUG_ON(qcs->endp && !(qcs->endp->flags & CS_EP_ORPHAN));