feat: add policy for how we write IAM policy documents

[greenkiwi]

We want to use aws_iam_policy_document rather than json for our policy definitions.

resource "aws_iam_policy" "example_s3_access" {
  name        = "${module.component_metadata.full_name}-test-s3-access"
  policy = data.aws_iam_policy_document.example_s3_access.json
}

data "aws_iam_policy_document" "example_s3_access" {
  statement {
    actions = [
      "s3:ListBucket",
      "s3:GetObject",
      "s3:PutObject",
      "s3:DeleteObject",
    ]

    resources = [
      module.s3_bucket.s3_bucket_arn,
      "${module.s3_bucket.s3_bucket_arn}/*",
    ]
  }
}