Add support for database in library.

Feature Request

To support offline mutations even when the app is closed, we need to persist the mutation requests in the database.

• Implement a database (using SQLite or room) that stores the mutation requests made when the user goes offline.
• For this you have to understand:
  • How the interceptor works and what are the type of objects in the request.
  • Store that request in a database after serializing it accordingly.
  • Extract data from the database (de-serialize it) to form a request object when the network connection is regained and a call to the server can be made.

This link can help you in understanding the apollo interceptor which we have used in intercepting the mutation requests: https://github.com/apollographql/apollo-android/blob/master/apollo-runtime/src/main/java/com/apollographql/apollo/interceptor/ApolloInterceptor.java#L114