Add staging & testing rootDomain support for all validating functions in at_onboarding_flutter package

[yahu1031]

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

I'm trying to create a custom login screen. And trying to authenticate with

OnboardingService.getInstance().authenticate('@bob🛠', jsonData: atKeysFileData);

I'm running om the local server(vip.ve.atsign.zone). The authentication was failing with ResponseStatus.serverNotReached response cause _checkAtSignServerStatus('@bob🛠') returns ServerStatus.unavailable. This is because, The implementation of the function has AppConstants.rootDomain which is root.atsign.org. This won't let me check my local server atsigns.

Describe the solution you'd like

In the RootEnvironment extension for domain, assign values before returning them. And in case, If users want to have a custom login screen, they don't use any RootEnvironment. In such a case, the functions that use the rootDomain must be passed as an optional parameter.

NOTE:

If this wasn't modified as mentioned, then testing on local servers won't be possible for custom login screens.