[Feature Request] REST support

[nthings]

Hi! I was trying to use this emulator with a package we have that builds REST clients for GCP API's: https://github.com/goblet/goblet_gcp_client. Recently I added a feature here to support Pub/Sub and Spanner official emulators. But when trying to add this one I figured that does not support REST clients, just grpc. Would be nice that his handles REST calls using the grpc-gateway (https://github.com/grpc-ecosystem/grpc-gateway) the same way the official Spanner emulator does: https://github.com/GoogleCloudPlatform/cloud-spanner-emulator/blob/master/gateway/gateway.go I'm not that proficient on Go to do it myself but leaving the request here if somebody has the knowledge and time. Thanks in advance!