Phase 2 implementation for backup performance improvements

[sseago]

Describe the problem/challenge you have

This includes implementation of Phase 2, as described in #7474

Describe the solution you'd like

Building on the 1.15 changes which included the ItemBlock model and ItemBlockAction plugin type, this will enable multiple worker threads to process ItemBlocks in parallel.

Detailed design for this enhancement is included in the "Phase 2" section of https://github.com/vmware-tanzu/velero/blob/main/design/backup-performance-improvements.md

Anything else you would like to add:

Environment:

• Velero version (use velero version):
• Kubernetes version (use kubectl version):
• Kubernetes installer & version:
• Cloud provider or hardware configuration:
• OS (e.g. from /etc/os-release):

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