Fix latest version to properly support Product in plans

[remi-stripe]

This ensures that you can create plans with the latest API version. We had 2 separate bugs:

• re-using ProductParams meant that you could pass invalid parameters when creating a product inside a plan (such as Type which is unsupported).
• not properly enforcing the latest API version which means all requests are currently failing.

This is a breaking change but it fixes what we broke in 29.0 so it's likely a better fit for 29.0.2 but unsure.

r? @brandur-stripe
cc @stripe/api-libraries

[remi-stripe]

@brandur-stripe While on this PR, I think there's a bug in stripe-mock since it did not catch that product[type] on plan creation is forbidden.

[brandur-stripe]

Oops, I kind of skipped over this one. Thanks for taking Remi!

This is a breaking change but it fixes what we broke in 29.0 so it's likely a better fit for 29.0.2 but unsure.

Yeah .. I'm going to send it out as a patch release and make a note about 29.0. I doubt too many people were using the plan features of 29.0 yet, so hopefully it's okay.

@brandur-stripe While on this PR, I think there's a bug in stripe-mock since it did not catch that product[type] on plan creation is forbidden.

I think the problem currently is that extra/unknown parameters are allowed. In this particular product subschema, type isn't even registered as a possibility:

product:
  anyOf:
  - description: The product whose pricing the created plan will represent.
      This can either be the ID of an existing product, or a dictionary
      containing fields used to create a [service product](/docs/api#product_object-type).
    properties:
      id:
        type: string
      metadata:
        type: object
      name:
        type: string
      statement_descriptor:
        type: string
    required:
    - name
    title: inline_product_params
    type: object
  - description: The ID of the product whose pricing the created plan
      will represent.
    type: string

[brandur-stripe]

Released as 29.1.1.