Generate an OAuth Attach Token to pre-associate an OAuth flow with an existing Stytch User. Pass the returned oauth_attach_token to the same provider's OAuth Start endpoint to treat this OAuth flow as a login for that user instead of a signup for a new user.
Exactly one of user_id, session_token, or session_jwt must be provided to identify the target Stytch User.
This is an optional step in the OAuth flow. Stytch can often determine whether to create a new user or log in an existing one based on verified identity provider information. This endpoint is useful for cases where we can't, such as missing or unverified provider information.