[Fleet] Allow users to cancel inactive unenrollment actions

[criamico]

Follow up of https://github.com/elastic/kibana/issues/179399

Allow the user to cancel the scheduled inactive unenrollment actions defined in https://github.com/elastic/kibana/issues/179399.

• [ ] Cancellation should be possible between the time when agents are scheduled to be unenrolled and the time when we actually unenroll them
• [ ] Designs TBD

It could work as scheduled upgrades: when the unenroll bulk action batches are scheduled they show up on Agent activity and the users have a chance to cancel from the UI.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[kpollich]

@nimarezainia - I think this needs some definition + design. We should allow users to potentially intervene before automatic unenrollment occurs for their agents.

[nimarezainia]

@criamico is it technically possible to have a cancel action button in the agent activity flyout?

Currently I don't think our intention to un-enroll is shown anywhere. When the unenrollment has happened, that event gets recorded in the agent activity. But that is obviously too late. I think the watcher runs every 10minutes. I would say that as part of this issue we would first need to record our intention: "X agents will be unenrolled starting at " (time is 10 minutes from now), perhaps also pointing a the list of agents as we do for other activities in that flyout.

Once we have this message, then a cancel button would make sense. I don't know if there's anywhere else this can be displayed.

thoughts?