Used to migrate an active NET2GRID user to the SSO introspection flow. NET2GRID verifies that the user is active on NET2GRID platform using the access_token. Also verifies (via B2B communication with the external provider) that the provided external_provider_access_token is active and migrates the user by updating the external_authentication_provider_id provided by the external provider via introspection. The returned access_token (in the response body) is a token that can be used in CE-API.