Attribute profile support in metadata

There's an open issue relating to documenting support for particular
attribute profiles, either in metadata or inline within protocol messages.

I would initially suggest that we add an element to metadata within the
<AttributeAuthorityDescriptor> to carry profile URIs supported by the AA.
Something like <AttributeProfile> or similar, zero or more.

I don't think we (ok, I) understand enough about what we would do to
protocols yet to really see where it would go, short of embedding it in
every <Attribute> element.

-- Scott