Rename identities table columns for more clarity. Rename parameters,
arguments etc. to accommodate these changes.
Change that the SAML provider domain is persisted in the identities
table as the provider ID. Use the SAML Entity ID/Issuer ID of the
IdP instead.
Introduce saml identity entity (including migrations and a persister)
as a specialization of an identity to allow for determining the
correct provider name to return to the client/frontend and for assisting
in determining whether an identity is a SAML identity (i.e. SAML
identities should have a corresponding SAML Identity instance while
OAuth/OIDC entities do not).
Introduce a per provider configuration option that allows users to
define whether automatic linking of accounts on sign-up/sign-in is enabled
or disabled.
