Resolve "Feature Request - Data Source for handling Gitlab security-policies as code - Scan Execution Policy"

What does this MR do and why?

This datasource allows users to manage security policy policy.yml files via terraform datasource. This datasource will be used to feed into the gitlab_repository_file resource that will create the policy.yml file.

This MR is only for support of Scan Execution Policies, other policies will be provided support in future MRs.

Related Issues:

#6556

Example Usecase:

data "gitlab_security_policy_document" "scan" {
  scan_execution_policy {
    rules { ... }
    actions { ... }
  }
}

resource "gitlab_repository_file" "policy" {
  content = data.gitlab_security_policy_document.scan.yaml
}