Add utility function for generating preimage + payment hash for use with hold invoices

[rolznz]

Currently it's a bit long:

// Generate preimage and payment hash
const toHexString = (bytes) =>
  bytes.reduce((str, byte) => str + byte.toString(16).padStart(2, "0"), "")

const preimageBytes = crypto.getRandomValues(new Uint8Array(32))
const preimage = toHexString(preimageBytes)

const hashBuffer = await crypto.subtle.digest("SHA-256", preimageBytes)
const paymentHashBytes = new Uint8Array(hashBuffer)
const paymentHash = toHexString(paymentHashBytes)