Add support for runtime introspection in GraphQL Data Providers

[djhi]

Problem

See #9403
A GraphQL backend may support functionality that extends beyond the default Data Provider methods. One such example of this would be implementing GraphQL subscriptions and integrating ra-realtime to power realtime updates in your React-Admin application. However, graphql dataFrovider implementations can't have access to the introspection made by ra-data-graphql and may have to reimplement the same logic.

Solution

• Update ra-data-graphql so that the dataProvider has a getIntrospection method allowing custom implementations to leverage the introspection.
• Update ra-data-graphql so that the dataProvider has a client property allowing custom implementations to access the Apollo client easily whether it was provided or created by ra-data-graphql.

Additional Checks

• The PR targets master for a bugfix, or next for a feature
• The PR includes unit tests (if not possible, describe why)
  - [ ] The PR includes one or several stories (if not possible, describe why)
• The documentation is up to date