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.

servers/navigation_server_3d.h

@@ -56,6 +56,8 @@ public:
 	/// MUST be used in single thread!
 	static NavigationServer3D *get_singleton_mut();
 
+	virtual Array get_maps() const = 0;
+
 	/// Create a new map.
 	virtual RID map_create() const = 0;