feat(resource): add CreationTimestamp to GKECluster

[L3n41c]

Add the ability to filter GKECluster resources based on their creation timestamp the same way it is already possible for ComputeInstance.

The following configuration:

regions:
  - all

blocklist:
  - ""

resource-types:
  includes:
    - ComputeInstance
    - GKECluster

accounts:
  "datadog-agent-sandbox":
    presets:
      - cleanup

presets:
  cleanup:
    filters:
      ComputeInstance:
        - property: CreationTimestamp
          type: dateOlderThan
          value: "48h"
      GKECluster:
        - property: CreationTimestamp
          type: dateOlderThan
          value: "48h"

now generates the following output:

us-central1 - GKECluster - gcp-gke-bc54253 - [CreationTimestamp: "2025-08-20T12:28:58+00:00", Name: "gcp-gke-bc54253", Project: "datadog-agent-sandbox", Region: "us-central1", Status: "RUNNING", Zone: ""] - filtered: filtered by config