Add support for using GitHub app with automatic token refresh

[uroshercog]

Hello,

we started using this action and find it very useful except that if we use a GitHub App token it terminated after 1h because of the token expiration which cannot be changed.

This PR introduces a way to pass app credentials to the action and use the Octokit library which automatically refreshes the token in the back.

The implementation should be backwards compatible.

Happy to address any concerns because we'd really like this feature to land :)