Initiates an OAuth 2.0 authorization flow for unauthenticated users (sign-in or sign-up). Returns a provider authorization URL that the client should redirect to. After the user authenticates with the provider, they are redirected back to the redirectAfterAuth URL with tokens in the URL fragment.
The OAuth provider must be configured and enabled in the Replyke dashboard
before this endpoint can be used. The redirectAfterAuth URL must be in
the provider’s allowed redirect URI list.