fix: optional oneOf / allOf / anyOf / $ref's

[mnahkies]

previously, these would never be marked as optional, even if the key
they belonged to wasn't required

this was made evident whilst testing #109
which relies on oneOf to handle arrays of type

this moves that logic up into the AbstractSchemaBuilder, and applies it more predictably,
as well as swapping the order such that nullable always comes before optional

this means that schemas like:

schema:
  type: object
  properties:
    enabled:
      "$ref": "#/components/schemas/actions-enabled"
    allowed_actions:
      "$ref": "#/components/schemas/allowed-actions"
  required:
    - enabled

will correctly mark allowed_actions as optional, and schemas like:

setup_intent_payment_method_options:
  description: ''
  properties:
    us_bank_account:
      anyOf:
        - $ref: >-
            #/components/schemas/setup_intent_payment_method_options_us_bank_account
        - $ref: >-
            #/components/schemas/setup_intent_type_specific_payment_method_options_client
  title: SetupIntentPaymentMethodOptions
  type: object

correctly mark us_bank_account as optional.

note: the typescript types were already correct here. the problem wasn't picked up by typescript because were being stricter on the validation and then passing that to a looser type, eg: passing T to a T | undefined parameter.