conformance test

[disjukr]

https://github.com/protocolbuffers/protobuf/blob/main/conformance/README.md#testing-other-protocol-buffer-implementations

Testing other Protocol Buffer implementations

To run these tests against a new Protocol Buffers implementation, write a program in your language that uses the protobuf implementation you want to test. This program should implement the testing protocol defined in conformance.proto. This is designed to be as easy as possible: the C++ version is only 150 lines and is a good example for what this program should look like (see conformance_cpp.cc). The program only needs to be able to read from stdin and write to stdout.