Support backups from replica pod

[turbra]

What is the motivation or use case for the change?

pgbackrest/pgdumps backups are taken against primary pod.

Describe the solution you'd like

Enable an option to selectively trigger backups on replica, instead of primary.

Please tell us about your environment:

Where is this running ( Local, Cloud Provider): Local

• Storage being used: SAN
• PostgreSQL Version: 13.4
• Platform: Openshift
• Platform Version: 4.8.35
• Operator Version: 4.7.3
• Additional context: We are not using pgbouncer or pgpool

[andrewlecuyer]

@turbra thank you for the feature request and for using PGO!

We have an enhancement in our backlog to support backups from replicas as you described.