[IDEA] Support mTLS endpoint

[S-Bohn]

Is your feature request related to a problem? Please describe.
Grpcbin is used in nginx-ingress controller's e2e tests. For fixing a bug related to client-side mTLS authentication it would be helpful to also test this use-case. Therefore a grpc endpoint with mTLS authentication is required.

Describe the solution you'd like
Add a new endpoint (e.g. listening on port 9002) that requires the client to send a certificate during the TLS handshake.

Describe alternatives you've considered

Additional context
PR in nginx-ingress controller that requires a e2e test: kubernetes/ingress-nginx#8188 (comment)