Add SubtleCryptoProvider and update Webhooks to allow async crypto.

[dcr-stripe]

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r? @richardm-stripe

Summary

Introduce a new CryptoProvider which uses the SubtleCrypto API. This is the main implementation for the Web Crypto API.

This requires adding support for async HMAC computations in CryptoProvider and adding new async Webhook creation/verification functions which can use these.

As a result, we can now construct and verify a webhook asynchronously by doing:

const cryptoProvider = Stripe.createSubtleCryptoProvider();

const event = await stripe.webhooks.constructEventAsync(
  body, 
  signature, 
  secret,
  tolerance, 
  cryptoProvider
);

I opted to go down the route of having sync and async methods rather than parameterizing the CryptoProvider. We could have done something like CryptoProvider<T> and then constructEvent():T. However our internal code would still need to check whether the T was a promise or not. Instead this forces one to be explicit about whether you want to operate synchronously or asynchronously, and gives us an opportunity to error if trying to mix contexts.

Motivation

We need this for Deno support (#997). Deno's crypto package doesn't implement HMAC and instead relies on the Web Crypto API, which is all async.

Test Plan

Added tests throughout. Note that unfortunately the SubtleCryptoProvider can only be tested on Node 16, as Node only added a SubtleCrypto shim in Node 15. All tests are shared between sync and async versinos.

I've also set up a Deno template which processes webhooks asynchronously using this

[richardm-stripe]

nit: I try to avoid calling functions that call describe and it since this interacts poorly with test runners, but nbd

[richardm-stripe]

LGTM! I like the choice of completely parallel async and sync pathways.