K8SPG-869 make repo field completely optional

[gkech]

CHANGE DESCRIPTION

Problem:
repos option is currently required and this means that it cannot be completely commented out from the cr configuration.

If repos option is omitted with the existing approach, we get the following error:

 % ka deploy/cr.yaml
The PerconaPGCluster "cluster1" is invalid: 
* spec.backups.pgbackrest.repos: Required value
* <nil>: Invalid value: "null": some validation rules were not checked because the object was invalid; correct the existing errors to complete validation

Cause:
Short explanation of the root cause of the issue if applicable.

Solution:
Short explanation of the solution we are providing with this PR.

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?

[JNKPercona]

Test Name	Result	Time
backup-enable-disable	passed	00:10:55
custom-extensions	passed	00:15:26
custom-tls	passed	00:05:56
database-init-sql	passed	00:03:47
demand-backup	passed	00:22:23
finalizers	passed	00:04:12
init-deploy	passed	00:03:12
monitoring	passed	00:10:05
monitoring-pmm3	passed	00:08:18
one-pod	passed	00:05:49
operator-self-healing	passed	00:08:12
pgvector-extension	passed	00:02:54
pitr	passed	00:11:53
scaling	passed	00:04:58
scheduled-backup	passed	00:25:46
self-healing	passed	00:08:45
sidecars	passed	00:02:51
start-from-backup	passed	00:11:40
tablespaces	passed	00:06:49
telemetry-transfer	passed	00:03:42
upgrade-consistency	passed	00:08:20
upgrade-minor	passed	00:05:52
users	passed	00:04:48
We run 23 out of 23		03:16:45

commit: 2d8ba85
image: perconalab/percona-postgresql-operator:PR-1329-2d8ba854e