Initial bazel tests for C# protoc and grpc_protoc_plugin.
This initial test just generated code from the proto file and compares
the generated code against expected files.
I've put the tests in `test/csharp/codegen` as that is similar to where
the C++ tests are placed, but they could be moved to
`src\csharp` if that is a better place.
Further tests can be added once the initial framework for the tests is
agreed.
tony