fix(pubsub): respect gRPC dial option when PUBSUB_EMULATOR_HOST is set

[toga4]

Fixes #10039

This implementation was based on the following implementation of Spanner Client.

google-cloud-go/spanner/client.go

Lines 252 to 261 in 4d85153

	// Append emulator options if SPANNER_EMULATOR_HOST has been set.
	if emulatorAddr := os.Getenv("SPANNER_EMULATOR_HOST"); emulatorAddr != "" {
	emulatorOpts := []option.ClientOption{
	option.WithEndpoint(emulatorAddr),
	option.WithGRPCDialOption(grpc.WithInsecure()),
	option.WithoutAuthentication(),
	internaloption.SkipDialSettingsValidation(),
	}
	opts = append(emulatorOpts, opts...)
	}