Support isar store

[LasseRosenow]

As of now the graphql_flutter client only supports hive as a store by default.

But even the hive mentions in its README: "Consider using Isar, a Flutter database by the author of Hive that is superior in every way!"

So I was wondering, if graphql_flutter would like to implement more official stores like Isar for example.

[vincenzopalazzo]

we can take it in considederation