Add functionality to get all feature flags for a user

[jacobmoretti]

Adds and exposes a simple function for retrieving all feature flags for a user.

Usage may be along the lines of:

import { getAllFeatureFlags } from "react-launch-darkly";

....

export async function allFeatureFlags(launchDarklyClientKey, user) {
  const allFlags = await getAllFeatureFlags(launchDarklyClientKey, user);
  console.log(`allFlags: ${JSON.stringify(allFlags)}`);
  // Do something here with all the flags...
}

[toddw]

:+1: