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.