fetchAdminUserIds = async ({ userPoolId, adminsGroupName }) => {
  const admins = await pager.fetchUsersInCognitoUserPoolGroup({
    userPoolId,
    groupName: adminsGroupName,
    cognitoClient: exports.cognitoClient
  })
  return new Set(admins.map(admin => getCognitoUserSub(admin)))
}
Fetches the UserPoolIds of all users in the AdminsGroup.