Criteria for Attribute resolution

[Rod Widdowson]

I was looking at the notes from ACAMP and this phrase:

> Can attributes be used to drive some of the authentication processes?

Reminded me of https://issues.shibboleth.net/jira/browse/IDP-280 ("Define
Syntax for, and add parsers for the ActivationCriteria inside the Attribute
Resolvers").

As background, the code to this is already there in the Attribute Resolver
and we just have to work out how to configure it.

It occurs to me that as an easy way to at least get some experience with
this is extend the schema for  BaseResolutionPlugInType (which is the base
type of both the Attribute Definitions and the DataConnectors) to gain an
attribute called "activationCriterion" which is a reference to a bean ID
(defined somewhere else) which has to implement

Predicate<AttributeResolutionContext>

At least that way we have an easy way for people to add in the required
functionality even though it won't be pretty to start with.  Once we have
this we can look at what sort of Criteria we would like to specify out of
the box.

Thoughts?