Istio Config: Handle uniform parsing of ptype.struct

[mandarjog]

This is required for istio config

golang proto.jsonpb does not yet support Unmarshaling proto.Any and ptype.Struct
Other languages (python / Java / C++) do not have this problem.

gogo/protobuf#197
golang/protobuf#216
golang/protobuf#208

It can be worked around by using standard go json or yaml for decoders.

[mandarjog]

This is resolved by pr #189