pdu.h: Add Content-Format for application/ace+cbor

RFC 9200 defines the new media type application/ace+cbor for
Authentication and Authorization in Constrained Environments.  The
CoAP Content-Format value for this media type is registered as value
19.

include/coap3/pdu.h

@@ -221,6 +221,9 @@ typedef enum coap_pdu_signaling_proto_t {
 /* Content formats from RFC 8782 */
 #define COAP_MEDIATYPE_APPLICATION_DOTS_CBOR    271 /* application/dots+cbor */
 
+/* Content formats from RFC 9200 */
+#define COAP_MEDIATYPE_APPLICATION_ACE_CBOR      19 /* application/ace+cbor  */
+
 /* Note that identifiers for registered media types are in the range 0-65535. We
  * use an unallocated type here and hope for the best. */
 #define COAP_MEDIATYPE_ANY                         0xff /* any media type */