Support for gRPC health check endpoint

[ikhoon]

It would be useful to have a GrpcHealthcheckedEndpointGroup to use gRPC health checks in the Armeria gRPC client.

public class GrpcHealthcheckedEndpointGroupBuilder extends AbstractHealthCheckedEndpointGroupBuilder {

    protected abstract Function<? super HealthCheckerContext, ? extends AsyncCloseable> newCheckerFactory() {
        return new GrpcHealthCheckerFactory(...);
   }

}

[minkichoe]

Hi @ikhoon, I'd like to work on this issue as my first contribution to the project. Could you please let me know if it's okay to proceed?

[ikhoon]

For sure. 👍

[ikhoon]

Reference: https://grpc.io/docs/guides/health-checking/