Make max-backups configurable in LimitBased GC from Etcd CR

[anveshreddy18]

Feature (What you would like to be added):
Add maxBackupsLimitBased field to the spec.backup section of Etcd yaml.

Motivation (Why is this needed?):

• At present, there is no way to configure the number of full snapshots to retain in a limitBased GC policy as a default value of 7 is set in the code.
• This change will allow the user to configure the --max-backups flag of the etcd-statefulset thus allowing them to control the behaviour of Limit Based GC.

Note: It will be an optional field, when not set, max-backups will be defaulted to 7

Approach/Hint to the implement solution (optional):

Add the maxBackupsLimitBased field to the spec.backup section of Etcd yaml, and define the MaxBackupsLimitBased field in the BackupSpec struct of Etcd CRD