Give ReadOnlyAccess to this account to another AWS account. Replace <ACCOUNT_ID> by the AWS account ID you want to give access.

api.tf

data "aws_iam_policy_document" "instance_assume_role_policy" {
  statement {
    actions = ["sts:AssumeRole"]
    principals {
      type        = "AWS"
      identifiers = ["<ACCOUNT_ID>"]
    }
  }
}

resource "aws_iam_role" "api_gateway_cross_account_role" {
  name               = "apigateway-crossaccount-role"
  assume_role_policy = data.aws_iam_policy_document.instance_assume_role_policy.json
}

resource "aws_iam_role_policy_attachment" "sam_api_automation_policy_attach" {
  role       = aws_iam_role.api_gateway_cross_account_role.name
  policy_arn = "arn:aws:iam::aws:policy/ReadOnlyAccess"
}