feat(redis): added endpoints

[Mia-Cross]

Currently debugging private networks endpoints that will be used this way:

resource "scaleway_vpc_private_network" "pn" {
    name = "private-network"
}

resource "scaleway_redis_cluster" "mycluster" {
  version      = "6.2.6"
  node_type    = "MDB-BETA-M"
  user_name    = "user"
  cluster_size = 1
  password     = "Passw0rd/with/Number"

  private_network {
    private_network_id          = "${scaleway_vpc_private_network.pn.id}"
    service_ips = [
      "10.16.1.0/5"
    ]
  }
}

• Unrelated minor tests upgrade Added tests for overriding of the zone.