"As an API Developer, I want to support OAuth 2.0 authentication regardless of whether my API is implemented within MediaWiki or in a microservice, so that I can use the best tool for my job."

Optional, since the first API mounted (MW REST) is built inside MediaWiki and has direct access to OAuth through session management. It's likely that microservice-based APIs will be coming soon. At the very least, we should not prevent this user story from coming later. MediaWiki OAuth 2.0 access tokens are JWTs, so it should be possible for microservices to validate tokens directly by checking the signature.