fix: improve reliability of object cache management

[ermakov-oleg]

Summary

Port of upstream #508, #429

Fixes 5 bugs in ExtendedClient cache that made caching completely non-functional:

1. Wrong type assertion in isObjectCached: checked for *corev1.Secret instead of *pgbackrestv1.Archive, so Archive objects were never recognized as cacheable
2. GVK comparison in getCachedObject: compared objects via GetObjectKind().GroupVersionKind() which returns empty GVK for decoded objects — replaced with reflect.TypeOf() comparison
3. Inverted condition in removeObject: used != instead of == for GVK comparison, so cache invalidation never matched the right object — same fix with reflect.TypeOf()
4. Missing TTL initialization: cachedEntry.ttl was never set, defaulting to 0 — every cached entry expired immediately on next access
5. Incorrect unit comparison in isExpired(): time.Now().Unix() - fetchUnixTime returns seconds, but int64(ttl) yields nanoseconds — fixed with time.Since(time.Unix(fetchUnixTime, 0)) > ttl

Added 5 new unit tests (cache Archive objects, distinguish by type, removeObject matching, removeObject with shared keys, TTL initialization) and updated existing expired test to use real TTL.