LPS-48299 Introducing fetchExistingStagedModel method

Hey Brian,

This pull request introduces a new method called fetchExistingStagedModel to the SMDHs. It's purpose to implement a common logic on which we can rely and find models during the different phases of importing: validation, import, etc.

@danielkocsis is working on some additional changes regarding this area of the staging, he will send more PRs soon, we wanted to break the changes into more pieces.

Thanks,

Máté