Import/export GLTF extras to node->meta

This is useful for custom tagging of objects with properties (for example in Blender) and having this available in the editor for scripting.

- Adds import logic to propagate the parsed GLTF extras all the way to the resulting Node->meta
- Adds export logic to save Godot Object meta into GLTF extras
- Supports `nodes`, `meshes` and `materials` (in GLTF sense of the words)

tests/core/object/test_object.h

@@ -174,6 +174,31 @@ TEST_CASE("[Object] Metadata") {
 	CHECK_MESSAGE(
 			meta_list2.size() == 0,
 			"The metadata list should contain 0 items after removing all metadata items.");
+
+	Object other;
+	object.set_meta("conflicting_meta", "string");
+	object.set_meta("not_conflicting_meta", 123);
+	other.set_meta("conflicting_meta", Color(0, 1, 0));
+	other.set_meta("other_meta", "other");
+	object.merge_meta_from(&other);
+
+	CHECK_MESSAGE(
+			Color(object.get_meta("conflicting_meta")).is_equal_approx(Color(0, 1, 0)),
+			"String meta should be overwritten with Color after merging.");
+
+	CHECK_MESSAGE(
+			int(object.get_meta("not_conflicting_meta")) == 123,
+			"Not conflicting meta on destination should be kept intact.");
+
+	CHECK_MESSAGE(
+			object.get_meta("other_meta", String()) == "other",
+			"Not conflicting meta name on source should merged.");
+
+	List<StringName> meta_list3;
+	object.get_meta_list(&meta_list3);
+	CHECK_MESSAGE(
+			meta_list3.size() == 3,
+			"The metadata list should contain 3 items after merging meta from two objects.");
 }
 
 TEST_CASE("[Object] Construction") {