Ariadne 0.14.0

• Added on_connect and on_disconnect options to ariadne.asgi.GraphQL, enabling developers to run additional initialization and cleanup for websocket connections.
• Updated Starlette dependency to 0.17.1.
• Added support for multiple keys for GraphQL federations.
• Made Query type optional in federated schemas.
• Updated default resolvers to test for Mapping instead of dict.
• Removed ariadne.contrib.django. (Use ariadne_django instead).
• Updated query cost validator to handle optional variables.