Decouple fetchMore logic from ObservableQuery

[micimize]

We should extract the fundamentals of fetchMore into a helper function, so that a QueryResult from client.query can easily be used with it

https://github.com/zino-app/graphql-flutter/blob/6c188cbb1b287b2c1f7135c41ef4c0f1fe952886/packages/graphql/lib/src/core/observable_query.dart#L139-L198

related to #372

[micimize]

https://gist.github.com/micimize/cd51edf60d0c3f4cd04c2b78218890f1 is untested, but it should hopefully work for users not using ObservableQuery

[sokolej79]

Good decision, because now it is only available from client.watchQuery.
And if anybody use only client with own query widgets as I do it is impossible to get fetchMore from client.query and I don need client.watchQuery for ordinary infinity list scrolling.
Just note aside for flutter Query widget and why I dont use Query widget, it is rebuilding to often.
I think that is because of StreamBuilder, this line:
stream=observableQuery.stream
Stream should not be provided directly to StreamBuilder, that is why it is rebuilding to often. Should be "cached" onInit.

[micimize]

resolved in v4 some time ago:
https://github.com/zino-app/graphql-flutter/blob/7192c1669c7dbecff0bbcd0665522c5b41207ab2/packages/graphql/lib/src/graphql_client.dart#L187-L200