Add well-known types as fields in the compliance service

Well known types are listed in https://developers.google.com/protocol-buffers/docs/reference/google.protobuf, and their JSON representations are summarized in the bottom part of [this table](https://developers.google.com/protocol-buffers/docs/proto3#json).

We should have well-known types whose JSON representation is a string be fields in compliance.proto's `ComplianceData` to ensure clients serialize/deserialize them correctly.