Add offline database support

[Dahkenangnon]

• Add offline database support

Flutter mobile applications can have certain constraints to store data when the user is offline then replicate it to the feathers js server when the user is online.

The feature will look like:

1. Configuration

// Others stuffs
import 'package:shared_preferences/shared_preferences.dart';

// auto: true, meaning if you use client.services("messages").create(),
// the message is created automatically if the user is offline in local then enqueue
//  in flutter_feathersjs to be replicated to the server when a user came back online.

// If auto is false (default value), you must use instead of client.services("messages").create(), 
// client.services("messages").createOrSave() to use the feature.

client.configure(FlutterFeathersjs.store(SharedPreferences, auto: true));

2. Using

// If auto were configured to true, the package may automatically store data in local  
// Unless, you can do:

client.service('messages').createOrSave({
  text: 'A new message'
});

Some questions:

• static local database client for storing data?
• allow developers to choose their own local storage package?

PS: This is just an idea and we need your feedback before going deeper in implementation