Put secret between Kratos and API in the secrets file

[Hugo Tiburtino]

Since Kratos webhook has been failing (see repeated fixes), we rely now on frontend to do its job. Now, the secret to run the endpoint is done by a terraform randomizer resource and every time we need to create this resource again (it's rare, but it can happen), it will generate a new secret. We need to make this secret unchangeable, putting it in the terraform secrets file.