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?

[jacobg]

Any update on adding setDefaults?
@robingenz What workaround is there?

[robingenz]

PRs are welcome. The workaround is to just add a fallback in your JS implementation if no configuration value is returned.

[jacobg]

@robingenz How's this?
#970