feat: add openHiTLS backend support

- Add openHiTLS DTLS/TLS backend
- Integrate openHiTLS crypto into libcoap (D)TLS and OSCORE paths.
- Extend TLS tests and add interop validation script.

include/coap3/coap_dtls.h

@@ -78,6 +78,7 @@ typedef enum coap_tls_library_t {
   COAP_TLS_LIBRARY_GNUTLS,    /**< Using GnuTLS library */
   COAP_TLS_LIBRARY_MBEDTLS,   /**< Using Mbed TLS library */
   COAP_TLS_LIBRARY_WOLFSSL,   /**< Using wolfSSL library */
+  COAP_TLS_LIBRARY_OPENHITLS, /**< Using openHiTLS library */
 } coap_tls_library_t;
 
 /**