Add support for billing thresholds

[remi-stripe]

r? @mickjermsurawong-stripe @ob-stripe
cc @stripe/api-libraries

cc @clintonb-stripe Could you double check that I added the right set of properties too and I'm not missing any? (Java only lists properties, not parameters).

Docs: https://stripe.com/docs/billing/subscriptions/metered-billing/thresholds
Internal PR with gate removal: #126799

[clintonb-stripe]

Thanks for taking care of this! 🎉

[remi-stripe]

leaving approval to mick/ob since it's our process. Thanks a lot for the quick review @clintonb-stripe! Other two PRs incoming in a bit

[mickjermsurawong-stripe]

Thank you @remi-stripe!
There's one incorrect typing, and one is my proposal on shortening class nesting
ptal @remi-stripe

[mickjermsurawong-stripe]

Should this be typed as ThresholdReason?

[remi-stripe]

@mickjermsurawong-stripe I don't think so no. We don't do "list of constants" in java AFAIK. @ob-stripe do you agree?

[ob-stripe]

That's not what Mick meant. The threshold_reason attribute is an object, not a string.

[remi-stripe]

ohhh I misunderstood. That reason naming is throwing me off. Nice catch mick, fixing!

[mickjermsurawong-stripe]

Should the innermost class, when referred outside this class (in normal usage) will be
Invoice.ThresholdItemReason instead of the 3-level nesting as Invoice.ThresholdReason.ItemReason here? I think the former reads better.
So we can implement the following:

public static class ThresholdReason extends StripeObject {
  List<ThresholdItemReason> itemReasons
}
// the inner-most class is now at nested in Invoice instead
public static class ThresholdItemReason extends StripeObject {
}

Actually, in the autogen SDK, we currently enforce nesting at most two levels to flatten classes. It help to flatten long resource dependency we have in pay-server annotation too.
The Charge.Level3.LineItem will be in autogen (hypothetically) as Charge.Level3LineItem (although we decided on unwinding it to top level resource)

public class Charge {
  public static class Level3 extends StripeObject {
    List<Level3LineItem> lineItems;
    ....
  }
  public static class Level3LineItem extends StripeObject {
     ....
  }
}

[remi-stripe]

Fixed!

[remi-stripe]

@mickjermsurawong-stripe Fixed both! Can you take another look?

[mickjermsurawong-stripe]

yay! thank you @remi-stripe and @ob-stripe!
LGTM

[remi-stripe]

Had to rebase as it conflicted with #653

[remi-stripe]

tests passed, re-approving