Implement the component GIDUserAuthFlowController

The existing GIDSignIn implementation handles the both interactive and non-interactive authentication flow in a same method, which increase the code complexity. This refactor work will handle the interactive and non-interactive flow seperately. The component GIDUserAuthFlowController will handle the interactive user signin flow.

Start by reading the existing GIDSignIn implementation and tracing how it currently handles interactive and non-interactive authentication. Separate the interactive user sign-in flow into the GIDUserAuthFlowController component, leaving the two flows handled independently; the issue does not name specific files or tests.

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