Create new endpoint to create OAuth Access token

Create a new REST API endpoint that conforms with the rest of the API which replaces `POST https://github.com/login/oauth/access_token` ([Authorizing OAuth Apps](https://docs.github.com/en/enterprise-server@2.22/developers/apps/authorizing-oauth-apps#2-users-are-redirected-back-to-your-site-by-github))

The issue names no implementation files, tests, or entry points. First review the repository's scope and existing API conventions, then define the OAuth access-token endpoint so it replaces the referenced GitHub endpoint and conforms with the rest of the API.

索引モデルが issue の本文から書いたものです。