Fix ownership ref issue

[ahitacat]

Idea behind this solution

For referenced classes the idea is to preserve the links (version and service):

If we have a class that references to another service:

"my_service/v1_alpha1/my_class.model",
`
@ref(path="other_service/v1/my_class")
class MyClass {
}
`,
"other_service/v1/my_class.model",
`
class MyClass {
  link Foo []MyAttribute
  Loo MyAttribute
}`,

We would like that the metamodel reader conserves this attribute reference and that it builds it by pointing to the correct version/service. While at the same time change get the ownership of the attributes that are not linked.

So the my_class_type.go would look like:

type MyClass struct {
  foo  *v1.MyAttributeList
  loo   *MyAttribute
}

The builders of foo will point to the other_service/v1 and the builders of loo will point to my_service/v1_alpha1

Previous approach

Previous approach #210 was relying on ownership to know the service and version of type. Whoever this caused a problem with link types and nested type. A type could be link in a class and be an attribute of another class. This causes a conflict on preservation of link and it depended on how the tree representation was created, as it was in that moment where the ownership was modified.

Current solution

This work adds a new field in the attribute type linkOwner, that will preserve the link ownership if exists. When it is a link of a referenced class this field will held the version of the attribute. So, it is used in the reader and the builders to generate the sdk.

[nimrodshn]

Very nice! LGTM 💯