Correct serialization of google::protobuf::Any

From https://developers.google.com/protocol-buffers/docs/proto3#json

> If the Any contains a value that has a special JSON mapping, it will be converted as follows: {"@type": xxx, "value": yyy}. Otherwise, the value will be converted into a JSON object, and the "@type" field will be inserted to indicate the actual data type.

Currently Any is not special cased and is therefore not serialized/deserialized to/from this custom representation. This should be fixed

Start with the protobuf JSON mapping linked in the issue and trace how google::protobuf::Any is currently handled in this Rust generator. Compare the existing serialization and deserialization behavior with the documented @type/value representation. Done means Any supports the documented special mapping in both directions, with relevant tests added or updated.

Written by the indexing model from the issue text.