Allow target-specific overrides for default client transport-mode

Application can override the default client transport-mode from
mbed_app.json via definition:

"target.macros_add": ["MBED_CLOUD_CLIENT_TRANSPORT_MODE_UDP"],

If UDP or UDP_QUEUE is more suitable as default transport-mode.
For example cellular targets most likely should be UDP_QUEUE.

Author: teetak01

mbed_cloud_client_user_config.h

@@ -20,9 +20,13 @@
 #define MBED_CLOUD_CLIENT_USER_CONFIG_H
 
 #define MBED_CLOUD_CLIENT_ENDPOINT_TYPE         "default"
-#define MBED_CLOUD_CLIENT_TRANSPORT_MODE_TCP
 #define MBED_CLOUD_CLIENT_LIFETIME              3600
 
+// Application can override the default transport mode via `target.macros_add` definition in mbed_app.json.
+#if !defined(MBED_CLOUD_CLIENT_TRANSPORT_MODE_UDP) && !defined(MBED_CLOUD_CLIENT_TRANSPORT_MODE_TCP) && !defined(MBED_CLOUD_CLIENT_TRANSPORT_MODE_UDP_QUEUE)
+#define MBED_CLOUD_CLIENT_TRANSPORT_MODE_TCP
+#endif
+
 #ifdef MBED_CONF_MBED_CLIENT_SN_COAP_MAX_BLOCKWISE_PAYLOAD_SIZE
     #define SN_COAP_MAX_BLOCKWISE_PAYLOAD_SIZE    MBED_CONF_MBED_CLIENT_SN_COAP_MAX_BLOCKWISE_PAYLOAD_SIZE
 #else