Decides which local person a partner's verified subject may become: the person already linked to it, one to confirm and link, a new entry, or a name match (development only), under the relationship's subject policy and rules.
the federation relationship
the verified sign-in
the mapped attributes and username
{ ok: true, username, link, create, how, confirm }, or a refusal
{ ok: false, code, status, what, why, rule }
Forwards to
FederationSp.subjectDecision().