new resource for creating PG followers

[davidji99]

`https://data-api.heroku.com/graphql` POST (returns 200)

Authorization: Bearer <API_TOKEN>

{"query":"mutation AddPostgresFollower($addonUUID: ID!, $plan: String!, $region: String!) { addPostgresFollower(        leader_uuid: $addonUUID       plan: $plan        region: $region      ) {        id        name      }    }","variables":{"addonUUID":"ORIGINAL_UUID,"plan":"heroku-postgresql:private-0","region":"virginia"}}

{"data":{"addPostgresFollower":{"id":"UUID","name":"postgresql-round-#####"}},"extensions":{}}

[davidji99]

resource "herokux_postgres" "leader" {
   app_id = "APP_UUID"
   db_leader_id = "PG_UUID"
   plan = "standard-0"
   state = "leader"  
}

resource "herokux_postgres" "follower" {
   db_leader_id = herokux_postgres.leader.id
   plan = "standard-0"
   state = "follower"  
}

• state cannot be changed from "leader" to "follower" but the opposite is valid. If "follower" becomes "leader", this will trigger a "pg:promote".