K8SPG-595: set .spec.extensions.image field as required

[pooknull]

https://perconadev.atlassian.net/browse/K8SPG-595

DESCRIPTION

This PR sets .spec.extensions.image field as required

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?
• 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?

[hors]

@egegunes now even for builtin extensions we need to have image options, but I think it is ok for now

[JNKPercona]

Test name	Status
custom-extensions	passed
demand-backup	passed
init-deploy	passed
major-upgrade	passed
monitoring	passed
one-pod	passed
operator-self-healing	passed
pitr	passed
scaling	passed
scheduled-backup	passed
self-healing	passed
start-from-backup	passed
tablespaces	passed
telemetry-transfer	passed
upgrade-consistency	passed
upgrade-minor	passed
users	passed
We run 17 out of 17

commit: 29290f6
image: perconalab/percona-postgresql-operator:PR-818-29290f6b4