[Feature] Implement snapshot compaction

[shreyas-s-rao]

How to categorize this issue?

/kind enhancement

What would you like to be added:
Steward should provide the ability to compact a set of snapshots, comprising of one full snapshot and multiple delta snapshot, into one single full snapshot, so that it allows for faster restoration upon any DB corruptions. It is achieved by first restoring a snapshot set to a local embedded etcd, then performing an etcd compaction to remove old revisions, then defragmenting the etcd DB to minimize the size of the etcd (and the size of the snapshot to be uploaded), and then upload a full snapshot of this etcd to the same object storage container. Compaction can be run as a separate sub-command, so that it does not affect the regular functioning of steward backing the main etcd.

While the functionality of compaction remains similar to snapshot compaction provided by etcd-backup-restore, steward should re-use code from other packages to the maximum extent possible, in order to keep the compactor code as lean as possible.

Why is this needed:
Part of #1