[core] Support case where composed schema is single type definition

Created by: fantavlik

Description of the PR

Fix for https://github.com/OpenAPITools/openapi-generator/issues/2164

The general fix is to handle the case where a composed schema is a single element defining a type.

For example:

allOf/anyOf/oneOf:
  - type: string

^ this case is not really model composition but rather defining the type of the model which is equivalent to type: <>

Input:

components:
  schemas:
    RatType:
      anyOf:
      - type: string
      enum:
        - NR
        - EUTRA
        - WLAN
        - VIRTUAL
      type: string

Output for Go model_rat_type.go:

package openapi
type RatType string

// List of RatType
const (
	NR RatType = "NR"
	EUTRA RatType = "EUTRA"
	WLAN RatType = "WLAN"
	VIRTUAL RatType = "VIRTUAL"
)

@wing328 can you review this if you get a moment as it is a core change? I've tested Go and regenerated the sample files and there are no changes for Go at least, not sure about other languages.

cc: @mroland91 and @goyalnikhil