app.eachRepository: add `{ userToken }` option to iterate through repositories acccessible by user

[gr2m]

This would use this endpoint:
https://docs.github.com/en/rest/reference/apps#list-repositories-accessible-to-the-user-access-token

But I want to make sure that there is an actual usecase for iterating through all installation, and then check for repositories accessible by the user in each one of them with the user-to-server token.