Rewrite gRPC service invocation example

[artursouza]

Describe the proposal

Take the existing service invocation example and rewrite it to use the gRPC proxy approach instead:

https://github.com/dapr/java-sdk/tree/master/examples/src/main/java/io/dapr/examples/invoke/grpc

https://docs.dapr.io/developing-applications/building-blocks/service-invocation/howto-invoke-services-grpc/

[MregXN]

/assign