Conformance & metadata implications

Just occurred to me that we'll need to make sure that we align the metadata
schema to whatever decisions are reached on conformance. There may be
elements required in metadata that have to be made optional if particular
roles don't require support for profiles that were required before.

It may also be desirable, I guess, to introduce alternative role descriptors
that model different conformance profiles, rather than leaving it at just
SSO SP and IdP, but that may be unnecessary and would probably be best
avoided if we can.

-- Scott