Add missing "private" member to repositories in installation event

[gsaraf]

Event documentation shows an additional Private member not represented in the current schema. See: https://docs.github.com/en/developers/webhooks-and-events/webhook-events-and-payloads#installation

From playing with my own app, I found it exists. JSON looks like this:

  "action": "created",
  "installation": {
    "id": 00000000
    ...
  },
  "repositories": [
    {
      "id": 0000000000,
      "private": true
      ...
    }
  ],
  ...
}