`tools/importer-rest-api-specs`: support for parsing the Data Factory custom types of a `List[Reference]` into a regular Object Definition

[tombuildsstuff]

#4026 describes an issue where Data Factory is using a custom Swagger extension to define the types for fields, using Data Factory specific types (a combination of "type": "object" and "x-ms-format": "dfe-XXX").

Whilst #4237 supports parsing most of these types as regular Object Definitions, it doesn't do this for Lists of a Reference, which is what this issue is tracking.

This is because we need to know ahead of time the models which are available within the sub-directories, so that we can pull these in as-needed - and since there's some ongoing refactoring work in that area, I've intentionally opted to do this post the refactor. This is due to the majority of the benefit being for the other scenarios, and whilst this shouldn't be too hard to implement, shifting the logic around post-refactoring should be easier than trying to do this now.

Note this feature toggle currently disables the parsing of these, until this work is completed.