[Model] Add the ability to associate labels with a model

**Is your feature request related to a problem? Please describe.**
A need has been expressed where it would be useful to add labels to models, e.g. "LegalHasApproved" or "Experimental"

**Describe the solution you'd like**
To support this, we'll need to add a new table called `Labels`.
We'll also need to add a new method `public void AddLabel(Guid runId, string name, string value)`