Add opaque PSK identifier to mbedtls_ssl_handshake_params

This commit adds a field `psk_opaque` to the handshake parameter
struct `mbedtls_ssl_handshake_params` which indicates if the user
has configured the use of an opaque PSK.
diff --git a/include/mbedtls/ssl_internal.h b/include/mbedtls/ssl_internal.h
index 97abb9f..318d13f 100644
--- a/include/mbedtls/ssl_internal.h
+++ b/include/mbedtls/ssl_internal.h
@@ -283,9 +283,12 @@
     const mbedtls_ecp_curve_info **curves;      /*!<  Supported elliptic curves */
 #endif
 #if defined(MBEDTLS_KEY_EXCHANGE__SOME__PSK_ENABLED)
+#if defined(MBEDTLS_USE_PSA_CRYPTO)
+    psa_key_slot_t psk_opaque;          /*!< Opaque PSK from the callback   */
+#endif /* MBEDTLS_USE_PSA_CRYPTO */
     unsigned char *psk;                 /*!<  PSK from the callback         */
     size_t psk_len;                     /*!<  Length of PSK from callback   */
-#endif
+#endif /* MBEDTLS_KEY_EXCHANGE__SOME__PSK_ENABLED */
 #if defined(MBEDTLS_X509_CRT_PARSE_C)
     mbedtls_ssl_key_cert *key_cert;     /*!< chosen key/cert pair (server)  */
 #if defined(MBEDTLS_SSL_SERVER_NAME_INDICATION)