Add navigation source geometry parser callbacks

Adds navigation source geometry parser callbacks so that externals can hook their own geometry into the navigation mesh baking process.

servers/navigation_server_3d_dummy.h

@@ -182,6 +182,9 @@ public:
 	bool is_baking_navigation_mesh(Ref<NavigationMesh> p_navigation_mesh) const override { return false; }
 #endif // _3D_DISABLED
 
+	RID source_geometry_parser_create() override { return RID(); }
+	void source_geometry_parser_set_callback(RID p_parser, const Callable &p_callback) override {}
+
 	Vector<Vector3> simplify_path(const Vector<Vector3> &p_path, real_t p_epsilon) override { return Vector<Vector3>(); }
 
 	void free(RID p_object) override {}