autodect Map<String, ComplexType> from additionalProperties #11

extracted from on openapi-processor/openapi-processor-spring/issues/150

if #10 works and produces maintainable code try to automatically detect a Map<String, ComplexType> from additionalProperties without an explicit mapping:

# ...
type: object
additionalProperties:
  type: array
  items:
    type: string

or even more complex:

additionalProperties may contain an object that should be generated:

# ...
type: object
additionalProperties:
  type: object
  properties:
    foo:
      type: string
    bar:
      type: array
      items:
        type: string