Repo States: Extract read/write methods to repo states store

[norascheuch]

This is a first step to separate the repo-states domain and data model. This PR creates a new repo-states store that hosts the read- and write methods.

Checklist

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.