Add UsageRecord creation ability

[abouzek]

While working on a project utilizing the new metered/usage based billing, I noticed the library didn't support UsageRecords yet, despite the docs having Java examples (probably due to auto-generated docs based on the template):

https://stripe.com/docs/api/java#usage_record_create

This is not really what I would consider a 'final' or mergeable copy of work, but I wanted to throw it out there for anyone utilizing the new feature.

I had to add subscriptionItem ID as a parameter passed to create because Stripe API will throw a 400 if you are using a sub-object endpoint such as subscription_items/<subscription_item_id>/usage_records and you pass the subscription item ID in both the URL and the body params. I think this problem needs to be solved in a more generic way, but for the intent of moving forward in my current project I am leaving as is and am open for suggestions :)

[alexander-stripe]

Thank you very much for the pull request! I've just opened a new PR which includes all our flexible billing primitives including the usage record endpoints: #462 .

[abouzek]

Awesome, thanks @alexander-stripe. I'll go ahead and close this as it duplicates some of that work.