Allow inherited lists as reference types

[Dresel]

This addresses the issue that inherited lists (or custom implementations of IEnumerable<T>) cannot be used and will lead to runtime exceptions (e.g. #380).

Implementation Detail:

• Distinguish between framework implementations of enumerable (checking via Namespace?.StartsWith("System")) and custom / inherited enumerable implementations
• Extend the ListObjectTypeVisitor to produce a reference if a custom list is used (as an alternative we could implement another Visitor)
• Adding IsReferencedOpenApiArray checks where necessary

I added additional unit / integration tests to cover this use case.

Let me know what you think and if you favor changes / a different implementation. If accepted this approach could be applied for dictionaries also (#313), see proof of concept branch Dresel:v2-dictionary-as-reference-types.

[Dresel]

Let me fix those tests first 😄

[Qualizorg]

Can we accelerate this?