[FR]: setDefaults option without replace all

### Description

Add the possibility to update setDefaults without replace the old ones if keys aren't the same.

### How would you use it?
I'm using the [add-to-app ](https://docs.flutter.dev/add-to-app)implementation and the host app already has defaults values. But in the flutter side I want add more defaults and I don't want add these defaults in android app code.

I opened a issue in https://github.com/firebase/flutterfire/issues/17620 but firstly this feature must be available in the native SDK.

### API Proposal

```
(void)setDefaults:(nullable NSDictionary *)defaults
replace:(BOOL)replace;
```

if replace false the new parameters will be added with the old ones. If true, it will be replaced.

### Firebase Product(s)

Remote Config