K8SPG-615: Add initialDelaySeconds for backups

[egegunes]

CHANGE DESCRIPTION

Problem:
In some environments, backup pod can be scheduled to a node but might need some time to be able to reach API server.

Solution:
We're introducing a new field to delay starting backup after pod is scheduled. If user configures spec.backups.pgbackrest.manual.initialDelaySeconds, backup process will wait for configured time before connecting to API server and starting the backup. Sleep duration will be printed in backup container logs:

time="2025-04-09T08:40:28Z" level=info msg="sleeping for 30 seconds"

Helm chart PR: percona/percona-helm-charts#522

CHECKLIST

Jira

• Is the Jira ticket created and referenced properly?
• Does the Jira ticket have the proper statuses for documentation (Needs Doc) and QA (Needs QA)?
• Does the Jira ticket link to the proper milestone (Fix Version field)?

Tests

• Is an E2E test/test case added for the new feature/change?
• Are unit tests added where appropriate?

Config/Logging/Testability

• Are all needed new/changed options added to default YAML files?
• Are all needed new/changed options added to the Helm Chart?
• Did we add proper logging messages for operator actions?
• Did we ensure compatibility with the previous version or cluster upgrade process?
• Does the change support oldest and newest supported PG version?
• Does the change support oldest and newest supported Kubernetes version?