[security-services] Issue for future discussion: ID attributes

In the course of implementing signing support, I came to the conclusion
that one of the suggestions made in the past by Eve to support an XML ID
attribute for Request, Response, and Assertion would be a good idea.

There are lots of potentially uses in XML, for example the ability to
build links to these things with XPointers, which is useful in
simplifying the signing transforms.

I don't think it's necessary for this use of ID to overlap/merge with
the existing SAML ID attributes, though it could. This also can be added
post 1.0 without compatibility problems, so it's not a major issue.

-- Scott