[Reviewer: Mike] Support AKA auth in Homestead

Mike,

I think you'd expected to review this in the last sprint - push this back to me if you don't think you'll have time. This adds and tests a new /impi/<impi>/av/(digest|aka)? resource for retrieving full SIP Digest or IMS AKA authentication vectors.

It preserves the existing /digest URL unchanged for now, so is safe to merge if you're happy with it.