fix: Uptake at_client_mobile 3.2.18

[sitaram-kalluri]

- What I did

• Uptake at_client_mobile 3.2.18 changes into at_onboarding_flutter.

- How I did it

• Refactor "onboard" and "authenticate" methods to use "AtAuthService" instead of "AtClientService" which is introduced in at_client_mobile.
• In onboarding_service.dart, mark "atClientServiceMap" as deprecated. This is used in the wavi mobile apps. Perhaps might have its usage in other mobile apps also. Hence marking as deprecated and adding instance of "AtClientService" as value to prevent breaking change.
• Added "enroll" method to submit a new enrollment request

- How to verify it

• Manually tested with at_onboarding_flutter example and wavi mobile app.

[CurtlyCritchlow]

LGTM and the example app works. Unable to verify with wavi.