Add support for PendingUpdate and ProrationBehavior on Subscription APIs.

[cjavilla-stripe]

I don't feel super confident in the type for SubscriptionItems on PendingUpdate here and am just trying to wrap up testing of deserialization locally.

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

[remi-stripe]

Missing the new events too

[cjavilla-stripe]

@ob-stripe when you get a chance. I'm generally curious how you think we should handle the see cref references in the doc comments once we have auto-gen. Curious if we should always use <see cref="TitleCasedThing" />? or if we need to some how know when to use <see cref and <c> depending on context

[remi-stripe]

Minor nitick on the docs + the ones you didn't fix from earlier.

re-assign to @ob-stripe for review as there's some risk with which fields can be nullable on the resource and I never know how to assess this safely

[remi-stripe]

also it's missing the constants for the events right?

[cjavilla-stripe]

Good catch on events, @remi. I noticed it here: https://github.com/stripe/stripe-java/pull/925/files but not in the latest stripe-java PR: https://github.com/stripe/stripe-java/pull/933/files should that be in the latest?

I tried testing out the <see cref feature with visual studios and wasn't seeing how this actually works.

For instance, here's Plan.BillingScheme's docs, which don't seem to link to the properties?

[cjavilla-stripe]

updated and pretty sure I got the comment references this time.

r? @remi-stripe

[ob-stripe]

Couple of comments re. some nullable or non-nullable resource fields.

Could you also bump stripe-mock while you're at it? Thanks!

[cjavilla-stripe]

Flipped the nullability of those two columns and bumped stripe-mock. Thanks for looking @ob-stripe

r? @ob-stripe

[ob-stripe]

lgtm!