Support openapiV3 oneOf for fields/responses

[kkkiio]

Describe the PR
Adds support for the oneOf feature in OpenAPI v3, allowing you to describe a field/response that can have multiple types.

Relation issue
Close #1363
Close #1617

*Additional context
Example:

type Dog struct {
	Bark  bool   `json:"bark"`
	Breed string `json:"breed" enums:"Dingo,Husky,Retriever,Shepherd"`
}

type Cat struct {
	Hunts bool `json:"hunts"`
	Age   int  `json:"age"`
}

type Pet struct {
	PetType string `json:"pet_type" enums:"Cat,Dog"`
	PetDetail any `json:"pet_detail" oneOf:"Cat,Dog"`
}

// @Summary Get pet by ID
// @Param id path string true "ID"
// @Success 200 {object} Cat
// @Success 200 {object} Dog
// @Success 200 "Return Cat or Dog"
// @Router /pets/{id} [get]
func GetPetByID() {

}

[ubogdan]

@kkkiio Thanks for your contribution

[nhtuan0700]

Hi @kkkiio
I did like that but it returns:

"200": {
           "description": "Return Cat or Dog"
}

instead of oneOf

---

swaggo version: v1.16.3

[kkkiio]

Unfortunately, the PR was reverted due to a compile error. I may create a new PR to address this.

Please note that the oneOf feature is supported by OpenAPI v3, which is only available in the v2 branch of the project that hasn't been officially released yet.