terraform-aws-notify-slack: When passing in SNS Topic, there's a race condition if it's not already created

If the SNS topic passed in is NOT already created, the notify-slack module will fail. This is due to notify-slack’s data objects being resolved before my resource objects.

resource "aws_sns_topic" "my_sns" {
  name  = "my-sns"
}

module "notify_slack" {
  source  = "terraform-aws-modules/notify-slack/aws"
  version = "2.0.0"
  slack_channel        = "my_channel"
  slack_username       = "My SNS"
  slack_webhook_url    = "https://hooks.slack.com/services/<SNIP>"
  sns_topic_name       = aws_sns_topic.my_sns.name  #<<<<<<<
  create_sns_topic     = false                #<<<<<<<<<
  slack_emoji          = ":yuk:"
  lambda_function_name = "${var.name_prefix}-my-slack-lambda"
}

From what I can tell, the only usage of the data resource is to ensure that SNS exists before running, and then to craft the ARN. I’d suggest hand-crafting the ARN with the Topic Name instead. I can submit a PR if it would be helpful.