Adding documentation and validations in OnboardingService.dart

[sitaram-kalluri]

Which at_widget package you find issue with?

at_onboarding_flutter

On what platforms did you experience the issue?

Android, iOS, Windows, MacOS, Linux

What version are you using?

at_onboarding_flutter : 6.19

What happened?

1. Add documentation to the OnboardingService.authenticate method to clearly define its purpose and behavior.
2. Add parameter validations in OnboardingService.authenticate method for cramSecret and jsonData as they are mutual exclusive.
3. In the OnboardingService.onboard method, propagate the rootDomain from the AtClientPreferences to the rootDomain in the AtOnboardingRequest.

[XavierChanth]

OnboardService.onboard also needs better documentation, it has multiple responsibilities and they aren't clearly defined:

• Automatic onboarding from keychain
• Initial activation of atSign
• Traditional onboard of a specific atSign?

[XavierChanth]

My main goal is to obsolete this work, as with the introduction of the at_auth package, OnboardingService doesn't really have a good reason to exist. There's also a layer in at_client_mobile which is a wrapper around at_auth that also supports keychain.

The layer in at_onboarding_flutter which should exist is a service which adapts those layers in at_auth and at_client mobile to useful functions which directly map to the user flows for app onboarding, and provide useful progress updates (i.e. a progress stream) to the calling code rather than async results.