Add --restore-point flag to Greenplum backup-fetch

[usernamedt]

Adds the ability to restore onto the provided restore point.

Usage:

wal-g backup-fetch --restore-point [restore_point_name]
wal-g backup-fetch [backup_name] --restore-point [restore_point_name]
wal-g backup-fetch --target-user-data=[user_data] --restore-point [restore_point_name]

If the backup name is not provided, WAL-G will find the most appropriate backup. Otherwise, it will also perform the check if the provided backup has been created before the restore point.