When a SAML IdP is created on a multi-ingress domain (implying that
multi-ingress domains are configured in Spar) the domain is added as `domain`
field to that IdP's `extraInfo` (`WireIdP` type in Haskell.) To avoid confusion
in later lookups, at most one IdP can be configured per multi-ingress domain.
If multi-ingress is not configured or it's not configured for the specific
domain, no `domain` field gets added to the IdP. This guards against creating
multiple IdPs and then assigning them to multi-ingress domains. Thus, users who
don't use multi-ingress don't observe any change. This feature only opens the
door to later provide an IdP for a multi-ingress domain.
