Add waitForConfirmation() function to RN SDK for use with declareReceivedPayment and other update actions

[MantisClone]

Problem

The declarative payment tests and node.js quickstart use this function to confirm requests' updates. However, the RN SDK does not provide this function.

const waitForConfirmation = async (dataOrPromise) => {
  const data = await dataOrPromise;
  return new Promise((resolve, reject) => {
    data.on("confirmed", resolve);
    data.on("error", reject);
  });
};

Proposed Solution

Add this function to the request-client.js package so that builders don't need to define it themselves.