Generate models for composite schemas

[rafalkrupinski]

Composite schemas are those using oneOf, anyOf, allOf and not.

• each schema will be represented as a single class (no inheritance)
• only schemas that are either defined inline or referenced under the paths, responses or x-apiResponses elements will be source for model classes
• schemas will be processed recursively
• all properties from all schemas will be present, except from not schema, which properties should be excluded
• each fields attributes (type, required, nullable) will reflect all the schemas, using the least restrictive one.
• processing contradictory schemas will fail