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/test/tests.go

@@ -57,7 +57,7 @@ func (s *Suite[C]) TestCreateWithConfig(t *testing.T) {
 	store(t, b, restic.ConfigFile, []byte("test config"))
 
 	// now create the backend again, this must fail
-	_, err = s.Create(s.Config)
+	_, err = s.Create(*s.Config)
 	if err == nil {
 		t.Fatalf("expected error not found for creating a backend with an existing config file")
 	}