Phase 1 implementation for backup performance improvements

[sseago]

Describe the problem/challenge you have This includes implementation of Phase 1, as described in https://github.com/vmware-tanzu/velero/issues/7474

ItemBlock model, ItemBlockAction plugin type, and backup logic refactoring to make use of these.

What is not included is the worker thread changes to allow processing of multiple items at the same time.

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