Classes vs Interfaces?

[OiNutter]

First big difference I'm noticing with this compared to swagger-typescript-api is that it generates classes for the API models rather than interfaces. This seems to cause us some issues with required fields (as described in #50) but potentially avoids duplication when extending the class.

Interfaces would avoid the initialisation warnings but only provided a guide for which fields needs to be added when creating a class that implements them, causing duplication. Additionally they may not flag errors in initialisation of non-null properties with null values due to API errors.

Probably worth some thought and investigation as to pros and cons of each, and whether we pick one and be opinionated about it, or maybe give the user the option of one of the other.