Fix naming convention of FirebaseOptions properties

[benPesso]

Not really a feature request, but not really a bug.

The properties in the FirebaseOptions class do not adhere to Flutter's naming convention. For example: databaseURL, googleAppID, etc.:

(new) FirebaseOptions FirebaseOptions({
	String apiKey,
	String appId,
	String messagingSenderId,
	String projectId,
	String authDomain,
	String databaseURL,
	String storageBucket,
	String measurementId,
	String trackingId,
	String deepLinkURLScheme,
	String androidClientId,
	String iosClientId,
	String iosBundleId,
	String appGroupId,
	String googleAppID,
	String projectID,
	String bundleID,
	String clientID,
	String trackingID,
	String gcmSenderID,
})