Support CBT without Long-Term Retention of Snapshots

[xing-yang]

Is your feature request related to a problem?/Why is this needed

The goal of introducing CBT support is to improve the performance of backups. However, requiring long term retention of snapshots can cause performance degradation on some storage systems, therefore defeating the purpose of supporting CBT. To solve this problem, we propose to support CBT without long-term retention of snapshots.

Describe the solution you'd like in detail

A solution is described in this doc:
https://docs.google.com/document/d/10HW9-6tQLCwJtxR8EhyWJA4COtRmpGz3IbtLB3NagE0/edit?tab=t.0

Describe alternatives you've considered

Additional context

[xing-yang]

Discussed at the DP WG meeting on 8/20/25 and agreed we will use the base_snapshot_handle to replace the base_snapshot_name.

[carlbraganza]

Please note that with the proposed change of base_snapshot_name to base_snapshot_id in the GetMetadataDelta call, the sidecar can no longer validate that the base_snapshot_id corresponds to the same PersistentVolume as the `target