Support for enum descriptions (x-enum-descriptions)

Created by: jmini

With https://github.com/OpenAPITools/openapi-generator/issues/893#issuecomment-416617460 we have introduced support for the x-enum-varnames extension.

In a similar way x-enum-descriptions should be supported:

  weather:
    type: object
    properties:
      type:
        type: integer
        format: int32
        enum:
          - 1
          - 2
          - 3
        x-enum-descriptions:
          - 'Blue sky'
          - 'Slightly overcast'
          - 'Take an umbrella with you'
        x-enum-varnames:
          - Sunny
          - Cloudy
          - Rainy

The java generator could add some Javadoc on top of the enum value.