Simulator: Added remote host option (#15443)

* Added an option to the Simulator module to connect to remote Gazebo servers.

This is usefull when the Gazebo simulation is running on a different host than the PX4 instance.
For example, we are running instances of PX4 with a companion application in multiple Dockers, for swarming simulations, which then connect to a remote Gazebo server.

A "-t" input argument has been added and can be called from the rcS startup script as: simulator start -t "192.168.178.122" $simulator_tcp_port

* _tcp_remote_ipaddr defaults to nullptr. This way testing against nullptr it can be determined if localhost or remote host is desired by the user.
* Documented the option in usage().

Signed-off-by: Peter Blom <peterblom.mail@gmail.com>

src/modules/simulator/simulator.h

@@ -123,6 +123,7 @@ public:
 
 	void set_ip(InternetProtocol ip) { _ip = ip; }
 	void set_port(unsigned port) { _port = port; }
+	void set_tcp_remote_ipaddr(char *tcp_remote_ipaddr) { _tcp_remote_ipaddr = tcp_remote_ipaddr; }
 
 #if defined(ENABLE_LOCKSTEP_SCHEDULER)
 	bool has_initialized() { return _has_initialized.load(); }
@@ -201,6 +202,8 @@ private:
 
 	InternetProtocol _ip{InternetProtocol::UDP};
 
+	char *_tcp_remote_ipaddr{nullptr};
+
 	double _realtime_factor{1.0};		///< How fast the simulation runs in comparison to real system time
 
 	hrt_abstime _last_sim_timestamp{0};