Scripted Attribute resolution and legacy support.

[Tom Zeller]

> In V2 we document "requestContext " to be that monster with 110
> methods.  Until r4358 "requestContext " pointed to a
> net.shibboleth.attribute.resolver.AttributeResolutionContext, we now point
> to this by a variable called "resolutionContext".

Thanks.

> I leave it to you (Tom) to decide with a request that if you want to create
> a new module, could you do the doings to make it happen so I can check stuff
> into it?

Let's use the existing idp-attribute-resolver-impl module for now.

If it becomes clear we need a new module we can always do that later,
at reasonably low cost.