Add streams endpoint

[josedonizetti]

1. Explain what the PR does

This PR adds the stream endpoint to the grpc server, which closes the implementation of the v1beta1 of the grpc API. The user needs pass which policies to create a stream, and optionally he can filter with fields to get as a response with the field mask.

2. Explain how to test it

test streams:

func main() {
	kk := keepalive.ClientParameters{
		Time:                10 * time.Second, // send pings every 10 seconds if there is no activity
		Timeout:             time.Second,      // wait 1 second for ping ack before considering the connection dead
		PermitWithoutStream: true,             // send pings even without active streams
	}

	var opts []grpc.DialOption
	opts = append(opts, grpc.WithTransportCredentials(insecure.NewCredentials()))
	opts = append(opts, grpc.WithKeepaliveParams(kk))

	conn, err := grpc.Dial(":4466", opts...)
	if err != nil {
		log.Fatalf("fail to dial: %v", err)
	}
	defer conn.Close()

	client := pb.NewTraceeServiceClient(conn)

	stream, err := client.StreamEvents(context.Background(), &pb.StreamEventsRequest{
		Policies: []string{"signature-events", "test"},
	})
	if err != nil {
		log.Fatal(err)
	}
	for {
		event, err := stream.Recv()
		if err == io.EOF {
			break
		}
		if err != nil {
			log.Fatal(err)
		}
		fmt.Printf("receive event: %v", event)
	}
}

test fieldmask:

func main() {
	kk := keepalive.ClientParameters{
		Time:                10 * time.Second, // send pings every 10 seconds if there is no activity
		Timeout:             time.Second,      // wait 1 second for ping ack before considering the connection dead
		PermitWithoutStream: true,             // send pings even without active streams
	}

	var opts []grpc.DialOption
	opts = append(opts, grpc.WithTransportCredentials(insecure.NewCredentials()))
	opts = append(opts, grpc.WithKeepaliveParams(kk))

	conn, err := grpc.Dial(":4466", opts...)
	if err != nil {
		log.Fatalf("fail to dial: %v", err)
	}
	defer conn.Close()

	client := pb.NewTraceeServiceClient(conn)

	stream, err := client.StreamEvents(context.Background(), &pb.StreamEventsRequest{
		Policies: []string{"signature-events", "test"},
		Mask: &fieldmaskpb.FieldMask{
			Paths: []string{
				"name",
			},
		},
	})
	if err != nil {
		log.Fatal(err)
	}
	for {
		event, err := stream.Recv()
		if err == io.EOF {
			break
		}
		if err != nil {
			log.Fatal(err)
		}
		fmt.Println("receive event", event)
	}
}

3. Other comments

[yanivagman]

LGTM