K8SPG-907 Fix panic on JSON marshal if PITRestoreDateTime is not a pointer

[pcman312]

---

Problem:
Closes #1356
There's a panic when marshalling the PITRestoreDateTime struct to JSON if the variable isn't a pointer. See the issue for a full breakdown.

Cause:
The MarshalJSON function is on a pointer receiver so json.Marshal skips over it and calls the MarshalText function on metav1.Time instead which panics on a nil receiver.

Solution:
Moves MarshalJSON onto a non-pointer receiver which Go will use for both non-pointer and pointer variables. I also added unit tests for this marshalling. I did not look at any other objects in the package to see if they have a similar problem.

CHECKLIST

Jira
No JIRA cards were created or managed, just the github issue referenced above

Tests

• Are unit tests added where appropriate?

Config/Logging/Testability
I don't believe any of these apply to this PR

• 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?

[it-percona-cla]

All committers have signed the CLA.

[JNKPercona]

Test Name	Result	Time
backup-enable-disable	passed	00:09:42
custom-envs	passed	00:20:15
custom-extensions	passed	00:14:22
custom-tls	passed	00:05:07
database-init-sql	passed	00:02:23
demand-backup	passed	00:24:30
finalizers	passed	00:03:38
init-deploy	passed	00:02:48
monitoring	passed	00:07:35
monitoring-pmm3	passed	00:09:56
one-pod	passed	00:06:01
operator-self-healing	passed	00:08:05
pgvector-extension	passed	00:02:39
pitr	passed	00:11:22
scaling	passed	00:05:14
scheduled-backup	passed	00:27:58
self-healing	passed	00:10:14
sidecars	passed	00:02:41
start-from-backup	passed	00:11:04
tablespaces	passed	00:06:36
telemetry-transfer	passed	00:03:45
upgrade-consistency	passed	00:07:58
upgrade-minor	passed	00:05:55
users	passed	00:04:24

Summary	Value
Tests Run	24/24
Job Duration	01:36:29
Total Test Time	03:34:23

commit: bf7608b
image: perconalab/percona-postgresql-operator:PR-1368-bf7608b80

[hors]

@pcman312 thank you for your contribution