Create an interface to abstract the upstream IDP from the
authorize, IDP discovery, callback, choose IDP, and login
endpoints. This commit does not refactor the token endpoint,
which will be refactored in a similar way in the next commit.
To make the subject of the downstream ID token more unique when
there are multiple IDPs. It is possible to define two IDPs in a
FederationDomain using the same identity provider CR, in which
case the only thing that would make the subject claim different
is adding the IDP display name into the values of the subject claim.
