[firebase_config] setDefaults option without replace all

## What feature would you like to see?

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

Example:

```java
public Task setDefaultsAsync(@NonNull Map defaults, @NonNull boolean replace)
```
if `replace` false the new parameters will be added with the old ones. If true, it will be replaced.

## 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.