Add support for configuring VGS label key

[shubham-pampattiwar]

Thank you for contributing to Velero!

Please add a summary of your change

• Introduces a new server flag --volume-group-snapshot-label-key to allow users to override the default label key used for grouping PVCs into VolumeGroupSnapshots.
• Adds spec.volumeGroupSnapshotLabelKey to the Backup CRD to support per-backup customization.
• Default label key is velero.io/volume-group-snapshot if not specified via flag or spec.
• Adds unit tests.

Does your change fix a particular issue?

Fixes #8934

Related to sub-task of #8865

Please indicate you've done the following:

• Accepted the DCO. Commits without the DCO will delay acceptance.
• Created a changelog file (make new-changelog) or comment /kind changelog-not-required on this PR.
• Updated the corresponding documentation in site/content/docs/main.

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.

Project coverage is 59.57%. Comparing base (4c23f66) to head (10859ae).
Report is 21 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/cmd/server/server.go	0.00%	1 Missing ⚠️
pkg/controller/backup_controller.go	75.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8938   +/-   ##
=======================================
  Coverage   59.57%   59.57%           
=======================================
  Files         370      370           
  Lines       40278    40285    +7     
=======================================
+ Hits        23995    24000    +5     
- Misses      14780    14782    +2     
  Partials     1503     1503           

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[shubham-pampattiwar]

Review request @Lyndon-Li @blackpiglet @sseago

[sseago]

Acking this, although we may want to consider in a subsequent PR renaming the default from "velero.io/volume-group-snapshot" to "velero.io/volume-group" -- since the labeled PVCs constitute the volume group, not the snapshot.