## 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
"""
    Signup serializer when the account is created using social authentication.
    Pre-processes information not obtained from SSO provider to create organization.
    """
    organization_name: serializers.Field = serializers.CharField(max_length=128)
    email_opt_in: serializers.Field = serializers.BooleanField(default=True)
    def create(self, validated_data, **kwargs):
        request = self.context["request"]
        if not request.session.get("backend"):
            raise serializers.ValidationError(
                "Inactive social login session. Go to /login and log in before continuing.",
            )
        request.session["organization_name"] = validated_data["organization_name"]
        request.session["email_opt_in"] = validated_data["email_opt_in"]
        request.session.set_expiry(3600)  
        return {"continue_url": reverse("social:complete", args=[request.session["backend"]])}
    def to_representation(self, instance: Any) -> Any:
        return self.instance
class SocialSignupViewset(generics.CreateAPIView):
    serializer_class = SocialSignupSerializer
    permission_classes = (CanCreateOrg,)
class TeamInviteSurrogate: