Stripe 5.1.0

This release:

• Adds a helper method to verify a signature for a Request.
• Conforms StripeSignatureError to AbortError for useful messages inside the Stripe logs.

func handleWebhook(_ req: Request) throws -> EventLoopFuture<Response> {
     // ...
     try StripeClient.verifySignature(for: req, secret: "whs_1234", tolerance: 500) 
}