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.

Changes will affect the plugins as well, Dániel is currently preparing that part.


Thanks,

Máté