Reuse existing plugin manager for get/put volume info

[sseago]

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Please add a summary of your change

Reuse existing plugin manager for get/put volume info. backup.FinalizeBackup was initializing its own plugin manager in getVolumeInfos but cleaning up too early, causing a new plugin process to be initialized in putVolumeInfos which was never getting cleaned up.

This PR changes the workflow to pass in the already-initialized plugin manager from the backup finalizer controller, since there's no need to have a separate one for get/put volume info.

Does your change fix a particular issue?

Fixes #7925

Please indicate you've done the following:

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

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 58.82%. Comparing base (3bd8a7d) to head (dc286a3).

Files	Patch %	Lines
pkg/backup/backup.go	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8012      +/-   ##
==========================================
- Coverage   58.82%   58.82%   -0.01%     
==========================================
  Files         346      346              
  Lines       28901    28891      -10     
==========================================
- Hits        17002    16996       -6     
+ Misses      10467    10465       -2     
+ Partials     1432     1430       -2     

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