convert to gRPC with REST proxy

[cainlevy]

AuthN could support both gRPC and REST by converting the core service to gRPC and integrating https://github.com/grpc-ecosystem/grpc-gateway. This could make it easier to write backend clients in target languages while maintaining flexibility for frontend clients that don't want to bundle grpc-web (50kb).

[dhillondeep]

After some discussion it was decided that it is best to have a GRPC fork for this and you can check the fork at https://github.com/uptimize/authn-grpc