Metadata support: Supported resolver query semantics; Iteration

[Tom Zeller]

>> I *think* in Columbus we had discussed potentially having another
>> interface provide an API like: Iterable<EntityDescriptor> etc.
>> Resolvers that can could optionally support this API also. This could be
>> used for the basic iteration cases.
>
> That's what I recalled. And our time right now needs to be spent on what
> the IdP needs, so keeping things separate is a reasonable way to do that.

Having looked at idp-metadata-api today, a summary of where we are at
with metadata and the idp would be welcome. I was a little surprised
how empty it was, but then again not really.