csi: fix check of StagePublishBaseDir being subdirectory of MountDir

[allisonlarson]

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

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• Changelog Entry If this PR changes user-facing behavior, please generate and add a
  changelog entry using the make cl command.
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  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.

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Changes to Security Controls

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

[tgross]

MARKER_REVIEW_XQF27717

@themavik keep this slop debris out of our repo, please. If you want to contribute helpful comments on PR reviews, that's fine. But AI-generated reviews are absolutely unwelcome.

[allisonlarson]

@chrisroberts I had forgotten the changelog, so thats been updated along with the appropriate labels