Feature request: Please provide support google.protobuf.Struct for a Mixer data type

[apicl]

I previously present the design doc Istio Auth Attributes (http://goo.gl/9dokCX). We need to define request.auth.claims to be google.protobuf.Struct. Can someone please add google.protobuf.Struct as a mixer data type.

Note that google.protobuf.Struct is an open source proto. See https://github.com/google/protobuf/blob/master/src/google/protobuf/struct.proto

[geeknoid]

Supporting structs as a first class attribute type has considerable implications. Whereas Mixer generally ensures strong typing, this would introduce weak typing into our model. It would also likely require substantial syntax enhancements in our expression language.