Add ability to block signup or set an allowed domain

[davidspek]

Currently any user is allowed to sign up to trace shield. While registration can be disabled in Kratos, we are using that registration form for the initial administrator setup as well. What we can do is add a webhook endpoint that Kratos will query before finalizing the registration where we can check if registration is allowed because either there is no current org admin or because the user is from a trusted domain.