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

[aslotte]

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)

[aslotte]

@Brett-Parker added a hacktoberfest label to this issue in case anyone would like to have a stab at it :)
To elaborate, were we thinking that we would associate labels to a run or to a registered model in this case (or an experiment)?

[Brett-Parker]

@aslotte I was thinking of a registered model. This would then give an ability to filter based on label. Potentially a great feature for the UI side down the line.