feat: add default health check handler to grpc server

The default health check handler is added to the grpc server as part of the gRPC service implementation for Dapr. This change adds the function `defaultHealthCheckHandler()` which returns `nil` when called, and updates `newServer()` function to include `healthCheckHandler` as a field in the struct.

grpc/server.go

@@ -14,6 +14,10 @@ import (
 	"github.com/dapr/go-sdk/service/common"
 )
 
+func defaultHealthCheckHandler(ctx context.Context) error {
+	return nil
+}
+
 // server is the gRPC service implementation for Dapr.
 type server struct {
 	pb.UnimplementedAppCallbackHealthCheckServer
@@ -25,7 +29,8 @@ type server struct {
 
 func newServer(lis net.Listener, opts ...grpc.ServerOption) *server {
 	s := &server{
-		listener: lis,
+		listener:           lis,
+		healthCheckHandler: defaultHealthCheckHandler,
 	}
 
 	gs := grpc.NewServer(opts...)