coap_mbedtls.c: Add in Connection ID support (RFC9146)

Requires MbedTLS 3.2.0 or later and MBEDTLS_SSL_DTLS_CONNECTION_ID defined
(which is the default in later versions of MbedTLS).

include/coap3/coap_dtls_internal.h

@@ -37,6 +37,11 @@
 
 #define COAP_DTLS_RETRANSMIT_COAP_TICKS (COAP_DTLS_RETRANSMIT_MS * COAP_TICKS_PER_SECOND / 1000)
 
+/* For RFC9146 Connection ID support */
+#ifndef COAP_DTLS_CID_LENGTH
+#define COAP_DTLS_CID_LENGTH 6
+#endif
+
 /**
  * Creates a new DTLS context for the given @p coap_context. This function
  * returns a pointer to a new DTLS context object or @c NULL on error.