UDP: extended datagram context. Sometimes it is required to process datagram properties at higher level (i.e. QUIC is interested in source address which may change and IP options). The patch adds ngx_udp_dgram_t structure used to pass packet-related information in c->udp.

--- a/src/event/quic/ngx_event_quic.c
+++ b/src/event/quic/ngx_event_quic.c
@@ -1920,7 +1920,7 @@ ngx_quic_input_handler(ngx_event_t *rev)
         }
     }
 
-    b = c->udp->buffer;
+    b = c->udp->dgram->buffer;
 
     qc->received += (b->last - b->pos);