Add support for simple Scala Enumeration

[jakipatryk]

Currently the library doesn't support Enumeration, which is supported by OpenAPI: https://swagger.io/docs/specification/data-models/enums/.

This ticket is only about simple Scala Enumeration, without Value renaming. So for example it should support:

object Things extends Enumeration {
  type Thing = Value

  val Pizza, TV, Radio = Value
}

but not:

object ThingsWithRenaming extends Enumeration {
  type ThingWithRenaming = Value

  val Pizza = Value("PIZZA")
  val TV = Value("TV")
  val Radio = Value("RADIO")
}