Add support for Server Side Encryption for backups

Percona Backup for MongoDB supports Server Side Encryption with AWS KMS and with keys stored on the client side:

https://docs.percona.com/percona-backup-mongodb/details/storage-configuration.html#server-side-encryption

There are also questions on the forum about this functionality: https://forums.percona.com/t/unable-to-define-a-service-account-for-replica-set-or-sse-for-s3-backups/19798

Operator should support this as well to close the parity gap.