Backport of alloc_runner: stop sidecar tasks last into release/1.2.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #13055 to be assessed for backporting due to the inclusion of the label backport/1.2.x.

WARNING automatic cherry-pick of commits failed. Commits will require human attention.

The below text is copied from the body of the original PR.

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Closes #10340

This PR addresses a defect where sidecar tasks are stopped prior to their associated main task being successfully stopped. This can create problems in scenarios where the main task can take a long time to shut down, but still need their sidecars to be operational while they finish their shutdown task (e.g. Consul Connect proxy).

This fix includes the following changes:

• Adds the IsSidecarTask helper method.
• Refactors other helpers to use IsSidecarTask.
• Updates the doc string on the hasSidecarTasks method to correctly describe the behavior of the function.
• Modifies the second kill loop in allocRunner.killTasks to skip shutting down sidecar tasks. This allows the existing logic in allocHandler.handleTaskStateUpdates to shutdown sidecars after the main task has exited as it was designed to do.
• Renames a variable to avoid package import collision.

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