backend: let ParseConfig return a Config pointer

In order to change the backend initialization in `global.go` to be able
to generically call cfg.ApplyEnvironment() for supported backends, the
`interface{}` returned by `ParseConfig` must contain a pointer to the
configuration.

An alternative would be to use reflection to convert the type from
`interface{}(Config)` to `interface{}(*Config)` (from value to pointer
type). However, this would just complicate the type mess further.

internal/backend/rclone/backend_test.go

@@ -17,11 +17,11 @@ func newTestSuite(t testing.TB) *test.Suite[rclone.Config] {
 
 	return &test.Suite[rclone.Config]{
 		// NewConfig returns a config for a new temporary backend that will be used in tests.
-		NewConfig: func() (rclone.Config, error) {
+		NewConfig: func() (*rclone.Config, error) {
 			t.Logf("use backend at %v", dir)
 			cfg := rclone.NewConfig()
 			cfg.Remote = dir
-			return cfg, nil
+			return &cfg, nil
 		},
 
 		// CreateFn is a function that creates a temporary repository for the tests.