Incompatible with prost >= 0.12.0

The demo software compiles properly with prost=0.11.9 but as soon as I bump up the version to 0.12.0 or higher, the code does not compile.

The error is about the ProstMessage trait not being satisfied for the generated proto message.

  = note: the following trait bounds were not satisfied:
            `Ping: ProstMessage`
            which is required by `Prost<Ping>: Format`
note: the trait `ProstMessage` must be implemented

Note that I have verified that the compiled proto file is exactly the same as the one in the demo.

prost-types below 1.13.0 does not derive Copy for protobuf::Timestamp so if it is used in proto files, the code won't compile.