Add new EventNotificationHandler class for better thin event management

[xavdid]

Why?

The event notification handler code has been in the beta and private-preview branches since last year, but it's time to take it to GA!

This PR is entirely pre-approved code from previous PRs. It takes the static files from codegen, plus all the manual changes I made directly in beta to get everything working. It's stacked on top of the non-verified work from this week, too. Once this lands, the managed handler code should be in sync across all channels.

Notably, the static files that we used to share functionality are no longer sourced from codegen. Instead, they're now true manual files in master whose changes will flow into preview branches like normal.

What?

• add EventNotificationHandler class & supporting infrastructure (ability to copy a StripeClient, etc)
• add stripeClient methods for creating a handler bound to that client
• add example
• add tests

See Also

• https://stripe.dev/blog/event-notification-handlers-thin-events
• internal doc
• Initial PR: Add EventNotificationHandler #1653
• Update PR (adds non-verified handlers): Add non-verified methods to managed handlers #1878

Changelog

• We've been putting a lot of time into rethinking the event handling experience in the SDKs. This new class is the culmination of that effort.
• They're designed for a tight coupling with both StripeClient and the fully-typed nature of thin events. This delivers painless event destination upgrades, in-editor checks for common mistakes, and better code modularity.
• Now that we've released thin event notifications for v1 objects, these new handlers are our recommended path for all integrations using thin event notifications.
• See more detailed docs here: https://docs.stripe.com/webhooks/event-notification-handlers