feat: Use time_left_seconds to wait for users export job to complete

[ReubenFrankel]

The time_left_seconds property seems to be present when a job has a status of processing:

{
  "type": "users_export",
  "status": "processing",
  "format": "json",
  "fields": [
    {
      "name": "user_id"
    },
    {
      "name": "email"
    },
    {
      "name": "email_verified"
    },
    {
      "name": "username"
    },
    {
      "name": "phone_number"
    },
    {
      "name": "phone_verified"
    },
    {
      "name": "created_at"
    },
    {
      "name": "updated_at"
    },
    {
      "name": "identities"
    },
    {
      "name": "app_metadata"
    },
    {
      "name": "user_metadata"
    },
    {
      "name": "picture"
    },
    {
      "name": "name"
    },
    {
      "name": "nickname"
    },
    {
      "name": "multifactor"
    },
    {
      "name": "last_ip"
    },
    {
      "name": "last_login"
    },
    {
      "name": "logins_count"
    },
    {
      "name": "blocked"
    },
    {
      "name": "given_name"
    },
    {
      "name": "family_name"
    },
    {
      "name": "last_password_reset"
    },
    {
      "name": "locale"
    }
  ],
  "percentage_done": 2,
  "time_left_seconds": 162,
  "created_at": "2022-03-21T22:42:48.637Z",
  "id": "job_CCBtbR1e1bp5yfcg"
}

We can use this value to wait in a more intelligent manner, falling back to the config job_poll_interval_ms if not present (i.e. when queued or processing, where there is no time_left_seconds property).