Cocoapods, IOS dependencies update, and FCMPlugin.getAPNSToken

Changelog

• Replaced internal dependencies in favour of dependencies from cocoapods to avoid conflict with other, newer, firebase plugins;

• FIRApp.configure execution is avoided if it has already been ran by another plugin;

• Added FCMPlugin.d.ts for type defition;

• For the IOS, APNS token can now be retrieved by the new method:

FCMPlugin.getAPNSToken(
  function(token) {
    console.info("Retrieved token: "+token)
  },
  function(error) {
    console.error(error);
  }
);

Special thanks to bhargav (@bhargavas12) for requesting the feature and helping testing it.