trhooks: Add TaskStopHook interface to services

[endocrimes]

We currently only run cleanup Service Hooks when a task is either
Killed, or Exited. However, due to the implementation of a task runner,
tasks are only Exited if they every correctly started running, which is
not true when you recieve an error early in the task start flow, such as
not being able to pull secrets from Vault.

This updates the service hook to also call consul deregistration
routines during a task Stop lifecycle event, to ensure that any
registered checks and services are cleared in such cases.

fixes #5770

[notnoop]

The change lgtm.

It's quite reasonable to have Stop here deregister as a catch-all case as Stop() is called after alloc is marked as dead in

nomad/client/allocrunner/taskrunner/task_runner.go

Lines 525 to 531 in ee7803d

	// Mark the task as dead
	tr.UpdateState(structs.TaskStateDead, nil)
	// Run the stop hooks
	if err := tr.stop(); err != nil {
	tr.logger.Error("stop failed", "error", err)
	}

. This may lead to multiple deregistering invocation, but we already deal with that in PreKill and Exited cases.

It would be great to have an integration test that ensures that service is deregistered when prestart hook fails.

[notnoop]

Do you mean 6 and 7 here?

[github-actions]

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