Add cloud provider event parsing methods

[xavdid]

Why?

Stripe events have supported AWS EventBridge and Azure Event Grid for a while, but the SDK hasn't paved that path well.

Our webhook parsing logic is closely tied to signature validation, which isn't relevant for events from cloud providers (which don't need to be validated). So, we needed matching non-validating webhook parsing methods.

But we didn't want non-provider event destination users using them (they should still validate events!), so we parse at the payload level to detect whether it looks like a cloud-provider hook and error if not.

Now, users of cloud-based event providers will be able to parse those event bodies and use both Snapshot and Thin events without much trouble.

Note: our naming is a little all over the place with these right now, so the error messages are a little dicey. You'll note the introduction of "thin event notification", which is a term we're trying out on other surfaces. We're hoping to have this all cleaned up by the GA launch; ignore for now.

What?

• add cloud provider parsing methods to stripe clients
• centralize some event parsing logic
• adjust some error messages
• add tests

See Also

• stripe/stripe-java#2201
• DEVSDK-3085