Add authentication support in the gRPC server

Currently, all clients could connect to the gRPC server with any auth. It's not safe for the production environment. We could simply support username/password or other auth method.

See also:
* https://github.com/LinuxSuRen/api-testing/blob/master/cmd/server.go
* https://grpc.io/docs/guides/auth/#extending-grpc-to-support-other-authentication-mechanisms
* https://github.com/grpc/grpc-go/blob/master/Documentation/grpc-auth-support.md