Enable support for multiple schemas

[rkrishnasanka]

What would you like to be added:

It would be great if mutli-schema support was enabled by default in graphjin. Right now it restricts us to use a single schema. However we have functionality spread over multiple schemas so it would be great to get the graphql to resolve.

Why is this needed:

In our applications we have parts of the database schema that we want to ensure has no API generation and tables over mutiple schemas that end up having to serve the API for