Imports of example-gateway-server command

PathSynopsis
contextPackage context defines the Context type, which carries deadlines, cancellation signals, and other request-scoped values across API boundaries and between processes.
flagPackage flag implements command-line flag parsing.
github.com/golang/glogPackage glog implements logging analogous to the Google-internal C++ INFO/ERROR/V setup.
github.com/grpc-ecosystem/grpc-gateway/examples/internal/gatewayPackage gateway is an example of grpc-gateway server