openapi 3.1 - add support for defining schema type as a list

[chrisradek]

In Open API 3.0, the Open API schema type field could only be a single string value.

In Open API 3.1, the Open API schema type field can be an array of string values.

This is especially useful when defining enums that are backed by multiple types.

Take the following TypeSpec example:

enum PetType { asString: "dog", asNumber: 1}

Our Open API 3.0 emitter will emit an error because enum can be a number or a string, but Open API schemas only support a single type. (Side note: could we support this via anyOf with multiple enum subschemas for Open API 3.0?)

With Open API 3.1, we could emit this valid schema (which our Json Schema emitter already does):

PetType:
  type:
    - string
    - number
  enum:
    - dog
    - 1