Allow subscriptions.configure() to be called with a Config object from GaaMetering.init()

[everyplace]

This PR exposes a new initialization variable in GaaMetering.init(), swgInitConfig, which allows for any subscriptions configuration parameter to be passed to the auto-initializer in GaaMetering.

• When using GaaMetering, developers are expected to include swg.js and set it to initialize manually.
• swg.js initialization is handled by GaaMetering.init() if required.
• There are times passing a configuration parameter is required for the proper execution of swg.js, for example setting a paySwgVersion.

With this PR, the following code may now be used to specify a configuration during initialization:

     GaaMetering.init({
        shouldInitializeSwG: true,
        googleApiClientId: GOOGLE_API_CLIENT_ID,
        allowedReferrers: [
          'example.com',
          'test.com',
          'localhost',
          'google.com',
        ],
        userState: {},
        unlockArticle: () => {},
        showPaywall: () => {},
        handleLogin: () => {},
        handleSwGEntitlement: () => {},
        registerUserPromise: new Promise(() => {}),
        handleLoginPromise: new Promise(() => {}),
        publisherEntitlementPromise: new Promise(() => {}),
        swgInitConfig: {}, //configuration object matching the Config type that subscriptions.configure() expects.
      });