Add Bucket Lock immutable objects support for AWS S3

[ishan16696]

What this PR does / why we need it:
This PR enables backup-restore to handle AWS S3's object lock mechanism which will make snapshots taken by backup-restore immutable. It also adjusted the Restoration and garbage collector functionality to handle immutable snapshots for S3 object store.

Which issue(s) this PR fixes:
Fixes #777

Special notes for your reviewer:
These case I have tested:

For Restoration functionality

• Restoration functionality of backup-restore should work with buckets which don't have bucket/object lock enabled (Backward compatibility).
• Restoration functionality of backup-restore should work with buckets which have both versioned immutable as well as non-immutable, non-versioned snapshots present.
• Restoration functionality of backup-restore should work with buckets which have only immutable versioned snapshots(full as well as delta) present.

For Garbage collection of snapshots:

• Backward compatibility with buckets which don't have immutability enabled.
• GC should work with buckets which have both immutable as well as non immutable objects.
• GC should work with buckets which have only immutable objects.

Release note:

etcd-backup-restore now supports immutable objects for storage provider: AWS S3, provided by the [Bucket Lock](https://aws.amazon.com/s3/features/object-lock/) feature.

Snapshots garbage collection performed by etcd-backup-restore (if enabled) for S3 is performed only when the snapshots's retention period get expires.

Support for Bucket lock (S3) in etcd-backup-restore is backward compatible. For more info please refer to this doc: https://github.com/gardener/etcd-backup-restore/blob/master/docs/usage/immutable_snapshots.md

[gardener-robot]

@ishan16696 You need rebase this pull request with latest master branch. Please check.