POC: adds MlModelPackage

What does this MR do and why?

A POC adding MlModelPackages, using something simillar to STI https://api.rubyonrails.org/classes/ActiveRecord/Inheritance.html. These would allow us to better setup Packages::Package

An example:

p = Project.find_by(id: 1)
model = Packages::MlModelPackage.create(name: 'amodel', version: '1234', project: p)
model.ml_model?
Packages::MlModelPackage.all # returns only ml_model

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.