Add Navigation function to get all navigation maps

Added new function that returns all created navigation map RIDs from the NavigationServer. The function returns both 2D and 3D created navigation maps as technically there is no distinction between them.

(cherry picked from commit c0fed1d4e8)

modules/navigation/godot_navigation_server.h

@@ -83,6 +83,8 @@ public:
 
 	void add_command(SetCommand *command) const;
 
+	virtual Array get_maps() const;
+
 	virtual RID map_create() const;
 	COMMAND_2(map_set_active, RID, p_map, bool, p_active);
 	virtual bool map_is_active(RID p_map) const;