Add a way to access pull request submitter name in release notes

We have a workflow where we want do generate the release notes only from pull requests and mention the pull request author. I have found no way to do this yet...

No file or test is named. Start by tracing how pull-request-only release notes are generated and where pull-request metadata is exposed; done means the release-note template or workflow can access and render the submitter name, with coverage for that path.

索引モデルが issue の本文から書いたものです。