Feature: add possibility to create enum using oneof/const notation

[Tnemlec]

This PR tries to implement enum with labels as discussed in this Stackoverflow thread without using extra field x-enum-varnames that can change between each OpenAPI consumer.

With this PR, this openapi definition:

Definition 👇

{
    "info": {
        "title": "My API",
        "version": "development"
    },
    "openapi": "3.0.0",
    "components": {
        "schemas": {
            "FindOneRequest": {
                "type": "object"
            },
            "FindOneResponse": {
                "properties": {
                    "user": {
                        "$ref": "#/components/schemas/User"
                    }
                },
                "type": "object"
            },
            "User": {
                "properties": {
                    "id": {
                        "type": "string"
                    },
                    "status": {
                        "$ref": "#/components/schemas/UserStatus"
                    }
                },
                "type": "object"
            },
            "UserStatus": {
                "format": "int32",
                "oneOf": [
                    {
                        "const": 0,
                        "format": "integer",
                        "title": "NOT_SET",
                        "type": "const"
                    },
                    {
                        "const": 1,
                        "format": "integer",
                        "title": "ACTIVE",
                        "type": "const"
                    },
                    {
                        "const": 2,
                        "format": "integer",
                        "title": "INACTIVE",
                        "type": "const"
                    },
                    {
                        "const": 3,
                        "format": "integer",
                        "title": "DELETED",
                        "type": "const"
                    }
                ],
                "type": "integer"
            }
        }
    },
    "paths": {
        "/users/{ID}": {
            "get": {
                "operationId": "UserFindRequest",
                "parameters": [
                    {
                        "in": "path",
                        "name": "ID",
                        "required": true,
                        "schema": {
                            "type": "string"
                        }
                    }
                ],
                "requestBody": {
                    "content": {
                        "application/json": {
                            "schema": {
                                "$ref": "#/components/schemas/FindOneRequest"
                            }
                        }
                    }
                },
                "responses": {
                    "200": {
                        "content": {
                            "application/json": {
                                "schema": {
                                    "$ref": "#/components/schemas/FindOneResponse"
                                }
                            }
                        }
                    }
                }
            }
        }
    }
}

Will be generated as:

// Defines values for UserStatus.
const (
	ACTIVE   UserStatus = 1
	DELETED  UserStatus = 3
	INACTIVE UserStatus = 2
	NOT_SET  UserStatus = 0
)

// UserStatus defines model for UserStatus.
type UserStatus = int32

I hope you find this useful ! Don't hesitate to suggest changes 👍