Add typed createFetchHttpClient function.

[dcr-stripe]

Notify

r? @richardm-stripe

Summary

Adds a createFetchHttpClient function similar to createNodeHttpClient and expose this on the Stripe object. This lets us type the client creation and eliminate an extra import in code while also not forcing all code to require the FetchHttpClient unless needed.

Implementation change

This changes:

const Stripe = require("stripe");
const { FetchHttpClient } = require('stripe/lib/net/FetchHttpClient')

const stripe = Stripe("...", {
  httpClient: new FetchHttpClient()
});

to:

const Stripe = require("stripe");

const stripe = Stripe("...", {
  httpClient: Stripe.createFetchHttpClient(fetch),
});

Test plan

Updated typescript tests. Also confirmed that this doesn't interfere with regular node implementations by cloning https://github.com/stripe-samples/accept-a-payment/tree/main/custom-payment-flow/server/node-typescript, pointing it to this branch, and making sure typescript compiles.