feat: allow use of existing lambda_role vs creating it

[heathsnow]

Description

This module calls the terraform-aws-lambda module which takes an input variable lambda_role for use if you don't want the module to create the role.

This PR simply passes through the option of setting lambda_role and, if it's set (ie not an empty string), not to create the role but to use the set value instead.

Motivation and Context

Many organizations require IAM creation to occur under a different user/role than that which deploys other types of resources. Many/most community modules support passing your own IAM roles as an option to the module creating it for this reason.

Breaking Changes

The logic I used (create_role = var.lambda_role == "") is backwards compatible and doesn't have any breaking changes.

How Has This Been Tested?

I tested this using my forked branch and calling the module like so:

module "rds-unisem_aoi_yield_dw-alarms_slack" {
  # source            = "terraform-aws-modules/notify-slack/aws"
  # use fork source (below) until module source (above) merges fork.  The fork allows you to
  # pass in your own lambda_role instead of the module creating it.
  source            = "git::https://github.com/heathsnow/terraform-aws-notify-slack?ref=lambda_role"
  lambda_role       = data.terraform_remote_state.iam.outputs.lambda_slack_notifier_arn
  sns_topic_name    = "my_sns_topic_name"
  slack_webhook_url = "https://hooks.slack.com/services/foo"
  slack_channel     = "my-alerts"
  slack_username    = "my_alert"
}

[antonbabenko]

Thanks @heathsnow !

v4.3.0 has been just released.

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues. If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.