fix(exp_modem): DTE should own both command and data terminal

reworks to clear shared_ptr<> of both command and data terminals
and having DTE own it. When we swith to CMUX mode the ownership is transfered to CMux terminal

components/esp_modem/include/cxx_include/esp_modem_dce.hpp

@@ -40,6 +40,7 @@ public:
     modem_mode get();
 
 private:
+    bool set_unsafe(DTE *dte, ModuleIf *module, Netif &netif, modem_mode m);
     modem_mode mode;
 
 };