Creates a new AuthManager instance using saved authentication settings.
The authentication and client configuration.
Ensures the access token is valid and refreshes it if expired.
A promise that resolves to a valid access token.
Exchanges an authorization code for access and refresh tokens.
The authorization code received from the OAuth flow.
A promise that resolves when tokens have been successfully obtained.
Returns updated authentication-related settings that can be saved.
A partial settings object containing updated tokens and expiry.
Manages OAuth2 authentication for accessing the Google Contacts API. Handles exchanging authorization codes, refreshing access tokens, and providing updated credentials.