Backport of csi: fix check of StagePublishBaseDir being subdirectory of MountDir into release/2.0.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #27717 to be assessed for backporting due to the inclusion of the label backport/2.0.x.

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

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Description

Small fix, adds an IsSubdirectory helper that checks if a path is a subdirectory of another using the filepath.Rel, instead of only checking for a prefix match of two strings. Prevents an error being thrown if the StagePublishBaseDir and MountDir are given as separate directories that still start with the same characters (like /csi and /csilocal)

Testing & Reproduction steps

Bug was found when using a csi_plugin block such as:

csi_plugin {
  ...
  mount_dir              = "/csi"
  stage_publish_base_dir = "/csilocal"
}

returned the errorCSIPluginConfig StagePublishBaseDir must not be a subdirectory of MountDir, now it does not error. Tests have been added/updated.

Links

Contributor Checklist

• Changelog Entry If this PR changes user-facing behavior, please generate and add a
  changelog entry using the make cl command.
• Testing Please add tests to cover any new functionality or to demonstrate bug fixes and
  ensure regressions will be caught.
• Documentation If the change impacts user-facing functionality such as the CLI, API, UI,
  and job configuration, please update the Nomad product documentation, which is stored in the
  web-unified-docs repo. Refer to the web-unified-docs contributor guide for docs guidelines.
  Please also consider whether the change requires notes within the upgrade
  guide. If you would like help with the docs, tag the nomad-docs team in this PR.

Reviewer Checklist

• Backport Labels Please add the correct backport labels as described by the internal
  backporting document.
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  in the majority of situations. The main exceptions are long-lived feature branches or merges where
  history should be preserved.
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  within the public repository.

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

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Overview of commits

• a4f577f