Standardize UsageRecord methods

[ob-stripe]

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

Replace UsageRecord::create($params) (where $params must include a subscription_item key) with SubscriptionItem::createUsageRecord($id, $params).

[remi-stripe]

Approving but technically it'd be better to add the new way to master and update the docs and such though

[ob-stripe]

Approving but technically it'd be better to add the new way to master and update the docs and such though

Good point. I've added the method to the current branch and released it as 6.43.0. Will close this PR and amend #692 to remove UsageRecord::create.