feat(remote-config): support default values

[robingenz]

Is your feature request related to a problem? Please describe:

Default values are currently not officially supported and have to be implemented with a workaournd.

Describe the solution you'd like:

It would be great if you don't need this workaround and can tell Firebase Remote Config the default values. Add a FirebaseRemoteConfig.setDefaults(...) method.

Describe alternatives you've considered:

Additional context:

[maylorsan]

Hi!Any updates here?