Implement an Artifact repository for the data access layer

[hbooth]

Use the repository pattern to create a repository for the Artifact Resource. This will be used to refactor parts of the Artifact service layer.

The MLFlow calls will be part of the Artifact repository. However we want to be able to inject the MLFlow client so that it can be mocked for testing.

Definition of Done:

• The Artifact repository is implemented
• The Artifact service layer is refactored to make use of the Artifact repository
• All existing tests pass
• The feature is merged into dev