Add custom action type to volume policies

[sseago]

Thank you for contributing to Velero!

Please add a summary of your change

This adds a new volume policy action type "custom", intended to be used by external plugins which handle PVCs.

Does your change fix a particular issue?

Fixes #9505

Please indicate you've done the following:

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

[kaovilai]

Will check later if this is ok. There's some areas that this need to not error for the new pending/lost PVC volume policy feature

[codecov]

Codecov Report

❌ Patch coverage is 9.09091% with 80 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.50%. Comparing base (1315399) to head (ef8ffdb).
⚠️ Report is 31 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/volumehelper/volume_policy_helper.go	4.61%	62 Missing ⚠️
.../plugin/utils/volumehelper/volume_policy_helper.go	0.00%	17 Missing ⚠️
pkg/backup/actions/csi/pvc_action.go	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9540      +/-   ##
==========================================
- Coverage   60.60%   60.50%   -0.10%     
==========================================
  Files         386      387       +1     
  Lines       36389    36589     +200     
==========================================
+ Hits        22054    22140      +86     
- Misses      12758    12861     +103     
- Partials     1577     1588      +11     

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