Allow the use of aliases in type designator values

[gouttegd]

When a class has a type designator slot, currently the only ways to designate the effective type of an instance is to use either the name or the URI of the class (depending on whether the slot is String-typed or URI-typed).

We will need a way to also allow the use of arbitrary aliases, which could be provided either:

• by the alias slot on a derived class (LinkML formally allows the use of that slot in a class definition, though it is undocumented and the associated meaning is unspecified);
• by the mechanism envisioned in Optionally allow unrecognised designated type, fallback to base class linkml/linkml#3701 (or a similar mechanism, in any case something added to LinkML’s meta-model);
• explicitly by the client code.