add akka-projection-grpc shopping cart sample

[pjfanning]

• should be based on latest akka-projection release that is Apache licensed
• the docker based deployment works best if we get pekko-projection 2.0.0-M1 released first so that we don't need to depend on snapshot jars
• update the grpc.md to add the documentation related to the sample (again based on latest akka-projection release that is Apache licensed)