fetchUsernamesByUserId = async ({ userPoolId }) => {
  const users = await pager.fetchUsersInCognitoUserPool({
    userPoolId,
    cognitoClient: exports.cognitoClient
  })
  return new Map(users.map(user => [getCognitoUserSub(user), user.Username]))
}
Fetches all users in the given user pool, and returns a map of UserPoolId
(`cognito:sub`) to Username.