feat(at_auth): Added registrar service class

[Zambrella]

- What I did

• Created the RegistrarServiceBase interface to define the contract for the service.
• Implemented the RegistrarServiceImpl to handle API communications with the registrar.
• Added ValidatePersonResponse to parse responses for validation requests.
• Introduced RegistrarException to standardise error handling for failed API calls.
• Implemented HTTP request retry logic with configurable retries and delay.
• Added comprehensive unit tests using mocktail to validate API interactions.

- How to verify it
Run the test suite:

cd packages/at_auth
dart test test/registrar_test.dart

- Description for the changelog
Added registrar service to handle API communications with registrar