fs: define the optional interface SetMetadata and implement it in wrapping backends

This also implements backend integration tests for the feature

backend/chunker/chunker_test.go

@@ -36,6 +36,7 @@ func TestIntegration(t *testing.T) {
 			"GetTier",
 			"SetTier",
 			"Metadata",
+			"SetMetadata",
 		},
 		UnimplementableFsMethods: []string{
 			"PublicLink",