Function use in the IdP

[Rod Widdowson]

> >	SignatureValidationFilter can take an injected
> > Function<XMLObject, Set<String>>.  
> >
> > I suppose we might provide such a function?
> 
> It's probably not a crazy idea. Keep in mind that's not PKIX at
> runtime, but in verifying metadata, and I like to be as flexible as
> possible in that area.
> 

I'll add one.  Any suggestions for location? Up to know everything has been
in n.s.i.profile.context.navigate and it certainly doesn't belong there.
idp-core seems useful, but it hasn't got any appropriate packages.

/Rod