feat(federation): relay our federation identity on GET /platform/v1/federation/identity

The portal has to send partnerStatusLink in CreateFederation, and FM
rejects anything but its own configured value, so the operator cannot
type it. Relay FM's new GET /internal/identity (operator role) so the
portal can read it instead. FM can only answer 200 or 500 here; any
other status is outside its contract and becomes OEG's 502.