Add related_object field to event types, remove fetchRelatedObject() …

[ramya-stripe]

Why?

The fetchRelatedObject() method on the pulled event did not really work as it did not have access to the RequestSender. In the interest of time we will recommend users to use the info in the related_object field on the pulled event to determine which stripe method to call to get the related_object.

That related_object info was available on the pulled event but did not show up in types before. This PR adds that info.

What?

• Removed the fetchRelatedObject() method from the individual event types
• Added property related_object on the individual event types. The type of this property will be Stripe.Event.RelatedObject
• No need to manually maintain the events service as we no longer try to intercept the retrieve and list endpoints to add the fetchRelatedObject()
• Add the definition for Stripe.ThinEvent. This got missed in the previous PR to next-major-infra where we copied over code
• Add definitions for parseThinEvent() and parseSnapshotEvent. The docs content for the input params have been copied over from Webhook.constructEvent()

[ramya-stripe]

@helenye-stripe I wanted to add tests that test the type for column C has the data and related_object fields. But there is a broader problem with types that I have described in https://jira.corp.stripe.com/browse/DEVSDK-2192 which @xavdid-stripe is helping to figure out.

Added https://github.com/stripe/stripe-node/pull/2190/commits/6f72bb2afffe29387783a683d5e6df646523e4f0 - but like I said earlier this test will pass even without the changes in this PR

Can we unblock this PR?