Add support for const property

[githubjakob]

OpenAPI supports a const property to indicate that an attribute has exactly one possible value.

Pydantic model_json_schema automatically generates this from Literal fields that define a single value.

class MyResponseModel(BaseModel):
    foobar: Literal["baz"]

Flask-OpenApi3 should produce the following schema:

components:
  schemas:
    MyResponseModel:
      properties:
        foobar:
          const: baz
          title: Foobar
          type: string

I am using Flask-OpenApi3 to generate Typescript types from the API spec, and it would be great to have const so that the Typescript type could be a string literal instead of just a string.