Support App ID for status source

## Prerequisites:

* Is the functionality available in the GitHub UI? If so, please provide a link to information about the feature.
Yes
not super clear in the docs, but available in UI

* Is the functionality available through the GitHub API? If the functionality is available, please provide links to the
API documentation (https://developer.github.com/v3/) as well as the Octokit documentation (https://octokit.github.io/).
Yes, search `required_status_checks.checks[].app_id` in https://octokit.github.io/rest.js/v18

## New Feature

Please describe the desired new functionality:
In UI/API, we are able to choose which app is allowed to post certain statuses (or at least, which we accept for required status completion). Can we add this to safe-settings to ensure statuses come from gh actions

まず、safe-settings が必須ステータスチェックをどのように表現して適用しているかを確認し、そのフローを GitHub API のフィールド required_status_checks.checks[].app_id と比較します。設定で受け入れるステータスを指定した App ID に制限でき、その動作がテストでカバーされていれば完了です。

索引モデルが issue の本文から書いたものです。