Add task listing and canceling to REST API and CLI

[bert-laverman]

In some situations, we have seen Axon Server tasks remaining present while they are no longer relevant. A maintenance option to list currently active tasks and cancel individual tasks would be useful for maintenance.

The suggested approach is to add the REST API endpoints as experimental features that need to be explicitly enabled in the properties. The CLI will need to be able to "survive" a disabled state gracefully.