Support / document how to use restic encryption with OADP

1. Proposed title of this feature request
 - restic encryption support in OADP

2. What is the nature and description of the request?
 - Customer would like to be able to use the encryption functionality which is provided by restic.
 
 For reference:
 - https://restic.readthedocs.io/en/stable/070_encryption.html
 - https://github.com/vmware-tanzu/velero/issues/1053

3. Why does the customer need this? (List the business requirements here)
 - Customer would like to manage restic's encryption with OADP. 

4. List any affected packages or components.
 - Restic

5. How would you like to achieve this? (List the functional requirements here)
 - By setting this feature in the Backup CR

6. Is the customer able to assist in testing this functionality if implemented?
 - Yes