`allOf` singletons for extension objects

[coriolinus]

The OpenAPI schema does not perfectly map to the behavior we always want. For example, we might want to define a newtype which sometimes is read-only, or write-only. Because the readability and writeability are properties of a schema in OpenAPI, we can't directly express that.

Instead, we work around this with an allOf singleton pattern:

components:
  schemas:
    Id:
      type: string
      format: uuid
      x-newtype:
        pub: true

    Thing:
      type: object
      properties:
        id:
          readOnly: true
          allOf:
            - $ref: "#/components/schemas/Id"
      required:
        - id

    WriteableThing:
      type: object
      properties:
        id:
          $ref: "#/components/schemas/Id"
      required:
        - id

This should produce output like this:

pub struct Id(pub Uuid);

pub struct Thing {
    #[serde(skip_deserializing)]
    id: Id,
}

pub struct WriteableThing {
    id: Id,
}

• determine precisely what overrides we support in an allOf singleton
• documentation
• test case
• implementation