[Feature Request] Subscription onConnect and onDisconnect hooks

Hello,

It would be nice to have the ability to run a mutation or trigger a lambda function when the client connects or disconnects from the subscription Websocket.

Similar to [Apollo](https://www.apollographql.com/docs/apollo-server/data/subscriptions/#onconnect-and-ondisconnect)

Start by reading the linked Apollo onConnect and onDisconnect reference, then trace how AWS AppSync handles subscription WebSocket connections. Determine the supported behavior for triggering a mutation or Lambda on connect and disconnect; done means the lifecycle hooks are implemented with clear semantics and coverage for both events.

Written by the indexing model from the issue text.