Remove exposure of libcoap internals

Remove the exposure of libcoap internals from CoapNode APIs for the upper layers of EDDIE.

communication/include/CoapNode.h

@@ -12,6 +12,10 @@
 
 std::string get_local_node_ip();
 
+typedef enum {GET, POST, PUT, DELETE} method_t;
+
+method_t method_from_string(const std::string& input);
+
 class CoapNode {
 private:
     CoapClient *client = nullptr;
@@ -32,10 +36,10 @@ public:
 
     std::string discover();
 
-    int send_message(coap_pdu_code_t method, const char *dst_ip, const char *dst_port,
+    int send_message(method_t method, const char *dst_ip, const char *dst_port,
                      const char *dst_res, const char *query, const char *data);
 
-    coap_pdu_t *receive_message(int token, int max_timeout);
+    std::string receive_message(int token, int max_timeout);
 };
 
 #endif //EDDIE_COAPNODE_H