MINOR: proxy_protocol: Ingest PP2_TYPE_UNIQUE_ID on incoming connections

This patch reads a proxy protocol v2 provided unique ID and makes it
available using the `fc_pp_unique_id` fetch.
diff --git a/include/types/connection.h b/include/types/connection.h
index 0c2d960..30cb895 100644
--- a/include/types/connection.h
+++ b/include/types/connection.h
@@ -469,6 +469,7 @@
 	char *proxy_authority;	      /* Value of authority TLV received via PROXYv2 */
 	unsigned int idle_time;                 /* Time the connection was added to the idle list, or 0 if not in the idle list */
 	uint8_t proxy_authority_len;  /* Length of authority TLV received via PROXYv2 */
+	struct ist proxy_unique_id;  /* Value of the unique ID TLV received via PROXYv2 */
 };
 
 /* PROTO token registration */