Notify long running operations that they were cancelled

[jmartisk]

If a connection is lost while a long running query or mutation is still running, it might be useful to notify the application that the operation (maybe, it depends on the use case) doesn't need to continue. For example, SmallRyeContext could contain a boolean value, or allow registering an onCancellation callback for that.

For subscriptions, I think this is already done as the returned Multi will receive an onCancellation and the application can react to it.