Support POST /create-payment-intent

Interact with Stripe's payment_intents API endpoint.

I'm not committing the index.html that contains client-side code that interacts
with the /create-payment-intent endpoint, but it contains sensitive information,
so I'm omitting it for now.

TL;DR:
- Define POST /create-payment-intent endpoint
- Include envStripeAPIKey in Context record
- Define a top-level Stripe module for making API calls
- Define types and instances that align with Stripes request and response types
- Depend on the Req library: a higher-level library than http-client

website/sandbox/learnpianochords/src/server/API.hs

@@ -11,3 +11,6 @@ import qualified Types as T
 type API = "verify"
            :> ReqBody '[JSON] T.VerifyGoogleSignInRequest
            :> Post '[JSON] NoContent
+      :<|> "create-payment-intent"
+           :> ReqBody '[JSON] T.PaymentIntent
+           :> Post '[JSON] T.CreatePaymentIntentResponse