backup: make backup timeout configurable

[fanminshi]

Currently, etcd backup operator has a default non-configuration backup timeout of 60s. This can causing issue when retrieving and saving backup is more than 60s as seen in #1903.

TODOs:

• Define a new backup field to take in a backup timeout. api: add backup policy to take in a backup timeout. #1907
• Implement backup timeout in etcd backup operator. *: implement configurable backup timeout #1908
• Add ctx to BackupWriter.Write() method. *: support Writer with context #1910
• Manual testing plan.

[xiang90]

sgtm