Feature: Specify subset of permissions to grant to the token

[bradynpoulsen]

Overview

Inspired by tibdex/github-app-token, I propose that a permissions input is added to be able to configure a desired subset of permissions that should be issued to the token.

Example Usage

- uses: getsentry/action-github-app-token@v2
  with:
    app_id: ${{ secrets.APP_ID }}
    private_key: ${{ secrets.APP_PRIVATE_KEY }}
    permissions: |
      {"contents": "read"}

Implementation

Per octokit/auth-app.js, it appears that a permissions argument can be specified to the appOctokit.auth call. I believe having a permissions input that is a JSON map of desired permissions would be the easiest adoption of this function.