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.

El issue nombra la repository dispatch event API de GitHub y la documentación sobre permisos, pero no indica archivos del repositorio ni pruebas. Empieza leyendo las referencias enlazadas al endpoint y a los permisos, y después revisa las instrucciones de contribución del repositorio. Para darlo por terminado haría falta una propuesta concreta, aprobada por un maintainer, para permisos de dispatch-event con alcance limitado.

Escrito por el modelo de indexación a partir del texto del issue.