Add logic for handling added repositories to the GitHub App webhook handler

[jhrozek]

On adding a new repository we would receive a webhook message that a new repository was added. The message looks like this:

{
    "action": "added",
    "installation": {
        "id": 49896345,
        "account": {
            "login": "jakubtestorg",
            "id": 142222776,
            "node_id": "O_kgDOCHoluA",
            "avatar_url": "https://avatars.githubusercontent.com/u/142222776?v=4",
            "gravatar_id": "",
            "url": "https://api.github.com/users/jakubtestorg",
	     ...
        },
  ...
        "app_id": 864067,
        "app_slug": "jakub-s-local-minder-test",
        },
        "events": [],
        "created_at": "2024-04-22T14:16:52.000+02:00",
        "updated_at": "2024-04-22T14:16:53.000+02:00",
    },
    "repository_selection": "all",
    "repositories_added": [
        {
            "id": 790254972,
            "node_id": "R_kgDOLxpVfA",
            "name": "newrepo2",
            "full_name": "jakubtestorg/newrepo2",
            "private": false
        }
    ],

After receiving such request we need to:

• Check that we know the installation ID and infer the project ID
• Route the repositories from repositories_added or repositories_deleted to the appropriate handler. For deletion we already have handleEntityDeleteEvent, we need to add a similar handleEntityAddedEvent
• The handler would create or remove the repository record in the database
• In case the repository was added, also create the webhook. Once the repository is created, Minder issues an event to reconcile it with the profiles in that project.