[Feature] Schedule regular backup compaction

[amshuman-kr]

Feature (What you would like to be added):

With the newly introduced compaction command in etcd-backup-restore to asynchronously compact backups (latest full snapshot and its following incremental snapshots into a single full snapshot) in gardener/etcd-backup-restore#301, we should enhance etcd-druid to schedule the backup compaction at regular intervals to limit the number of incremental snapshots at any point in time and hence enhance backup restoration performance.

Motivation (Why is this needed?):

Complete the functionality for the issue #88.

Approach/Hint to the implement solution (optional):

etcd-druid's main controller may create a CronJob as part of it's reconciliation cycle. There is no need include the logic for selecting existing cronjobs based on spec.selector (of the Etcd resources) because of #186.

[amshuman-kr]

/assign @aaronfern
cc @abdasgupta