## Is your feature request related to a problem?

Currently if you login with SAML and you don't have an account on the PostHog instance one of two things happen: a) either a new account is created **if the email matches the domain whitelist** or b) you get turned down at the door. Yet after some thought, the point of SAML is that you manage user provisioning in a separate centralized system, so we shouldn't have the extra domain check. 

## Describe the solution you'd like

If a user logins through SAML and their account doesn't exist, create it automatically, regardless of whether it matches the domain whitelist.


## Describe alternatives you've considered

Keeping as is.

## Additional context

There's a complicating factor which is that SAML login is instance-based yet user signup is organization-based, so there's the edge case of an instance having multiple orgs. Should the user then be signed up to all organizations?

#### *Thank you* for your feature request – we love each and every one!

posthog/api/signup.py
if user:
        return {"is_new": False}
    backend_processor = "social_create_user"
    user_email = details["email"][0] if isinstance(details["email"], (list, tuple)) else details["email"]
    user_name = (
        details["fullname"]
        or f"{details['first_name'] or ''} {details['last_name'] or ''}".strip()
        or details["username"]
    )
    strategy.session_set("user_name", user_name)
    strategy.session_set("backend", backend.name)
    from_invite = False
    invite_id = strategy.session_get("invite_id")
    if not user_email or not user_name:
        missing_attr = "email" if not user_email else "name"
        raise ValidationError(
            {missing_attr: "This field is required and was not provided by the IdP."}, code="required"
        )
    if not invite_id:
        domain_organization: Optional[Organization] = None
        
        if not settings.MULTI_TENANCY: