Expose the VolumeHelper to third-party plugins.

[blackpiglet]

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Does your change fix a particular issue?

Fixes #7818

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• Accepted the DCO. Commits without the DCO will delay acceptance.
• Created a changelog file or added /kind changelog-not-required as a comment on this pull request.
• Updated the corresponding documentation in site/content/docs/main.

[codecov]

Codecov Report

Attention: Patch coverage is 40.98361% with 36 lines in your changes missing coverage. Please review.

Project coverage is 58.80%. Comparing base (5c413ec) to head (1fd959d).

Files	Patch %	Lines
internal/resourcepolicies/resource_policies.go	3.70%	26 Missing ⚠️
pkg/backup/actions/csi/pvc_action.go	53.84%	4 Missing and 2 partials ⚠️
pkg/controller/backup_controller.go	50.00%	1 Missing and 1 partial ⚠️
.../plugin/utils/volumehelper/volume_policy_helper.go	88.23%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release-1.14    #7944      +/-   ##
================================================
+ Coverage         58.79%   58.80%   +0.01%     
================================================
  Files               345      346       +1     
  Lines             28766    28815      +49     
================================================
+ Hits              16914    16946      +32     
- Misses            10423    10438      +15     
- Partials           1429     1431       +2     

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