Makes a federation relationship's back-channel request to the URL in one of
its DIALLABLE attributes and reads the answer as JSON.
Another attribute is refused and logged under STS-FED-0046. Never
rejects.
the federation relationship
the attribute holding the URL, one of DIALLABLE
Optionaloptions: FetchOptions
method, form, bearer, basic and headers
a promise of { ok, status, json, text, url, errorCode, why }
Forwards to
FederationHttp.fetchJson().