Activation Criteria for Attribute resolvers

[Rod Widdowson]

 > I think we probably want an interface, a Scripted implementation of it,
> and that's about it.

I'm really not in a hurry to be doing this.  My suggestion is that we

- Move the tristate enum somewhere more general than Attribute Filter 
- Change the activation crtiterion to be a 
     Function<AttributeRecipientContext, Trisate> and define the interface
     Around that
- Config it by adding a 'criterion="SpringBeanRef"' attribute
- Build the scripted implementation

And leave it at that for 3.0

I think it would then be relatively easy to modify the relevant attribute
filters to implement that interface, but that could be for 3.1

If seems to gain traction I'll put a paragraph into the wiki to expand these
thoughts.