[REQ] support for "const" keyword

Description

OpenAPI's Schema Object for OAS 3.1 to use JSON Schema 2019-09, which includes the const keyword (In this PR: Defining constant value in response the introduction of this keyword was discussed)

Describe the solution you'd like

The OpenAPI Spec would looks like:

components:
  schemas:
    UserBody:
      type: object
      properties:
        kind:  
          type: string
          const: MyConstant
        id:
          type: integer
        firstName:
          type: string
        lastName:
          type: string

Generated source:

public class UserBody {
    public final Static String KIND = "MyConstant"
     //...

}

Describe alternatives you've considered

IMHO, define an enum attribute with a single value in the list is not the same.

Related issues

https://github.com/OpenAPITools/openapi-generator/issues/1916

Start with the OAS 3.1 Schema Object and the const keyword specification linked in the issue, then review the related OpenAPI discussion and issue 1916. Confirm how the const value should appear in the generated Java UserBody model and define completion across the affected generators and their tests.