Allow operator to pass in a list of managed IAM policy ARNs for the runner role

[jpalomaki]

For https://github.com/philips-labs/terraform-aws-github-runner/issues/308. Policies can be managed outside of this stack, or within this stack like so:

resource "aws_iam_policy" "my_policy" {
  ...
}

module "runners" {
  ...
  runner_iam_role_managed_policy_arns = [
    aws_iam_policy.my_policy.arn,
  ]
}

Tested by running a plan on an existing stack, with and without the list of managed policies.

[npalm]

@jpalomaki thanks looks all good

[npalm]

fix #308