Properly implement google.protobuf.Any json serde

The json representation for google.protobuf.Any is a nightmare: https://github.com/protocolbuffers/protobuf/blob/main/src/google/protobuf/any.proto#L100-L127

But we should probably implement it correctly.

The biggest issue is their notion of "well-known types with a custom json representation" - google.protobuf.Duration is mentioned, and it appears that "well-known" here refers to https://protobuf.dev/reference/protobuf/google.protobuf/, but I'm not sure how best to go about implementing this.