[Service] Allow aborting of tasks

Currently the only way for a user to stop a task is to ssh into the node running it and kill the cbbackupmgr process.

It would be nice to be allow to abort tasks vi the UI/Rest API. This endpoint should make sure that the user confirms the action they are doing. To avoid errors. We also should add a new error code to show that the task was cancelled by the user.

The aborting of tasks should be allowed not just for running tasks but tasks that are in queue to be run (status saying "waiting to run").