Apps: introduce a new "dispatch_event" permission

Creating [repository dispatch events](https://docs.github.com/en/free-pro-team@latest/rest/reference/repos#create-a-repository-dispatch-event) is a great way to create GitHub apps which can trigger actions in the repositories. For example, I can dispatch events for every repository in every installation of a registered app when a new release is created in a github repository. and the receiving repositories can act on that information as they please.

The problem is that the permission required for dispach events is [`contents:write`](https://docs.github.com/en/free-pro-team@latest/rest/reference/permissions-required-for-github-apps#permission-on-contents), which many users are understandably uncomfortable to give to a 3rd party application.

It would be great if apps could be granted a new permission which would

1. Only give access to dispatch events
2. Only permit to dispatch events from a list of event types

`1.` would already help greatly. `2.` would furthermore prevent one app to appear as another, by specifying exactly which event types one app is allowed to dispatch.

L'issue mentionne la repository dispatch event API de GitHub et la documentation sur les permissions, mais aucun fichier du dépôt ni aucun test. Commencez par lire les références liées à l'endpoint et aux permissions, puis examinez les consignes de contribution du dépôt. Pour considérer le travail comme terminé, il faudrait une proposition concrète, approuvée par un maintainer, concernant des permissions de dispatch-event à portée limitée.

Rédigé par le modèle d'indexation à partir du texte de l'issue.