Support LinkML Abstract classes

[jgadling]

LinkML supports an abstract: true property on classes. Our code generation should support these abstract classes, but there are some decisions we need to make first:

1. Do we want to represent these abstract classes in our DB schema or GraphQL schema at all, or do they just get hidden, with their fields inherited by concrete classes?

• The official LinkML -> SQLAlchemy codegen represents these abstract classes as separate tables/models using SA's concrete inheritance functionality (example here)

What we need to do:

• Decide how to map these abstract classes to SQLAlchemy models and/or GraphQL interfaces/types
• Implement the changes necessary to support the decision
• Document the rationale for our decision