Agent: Report Kubernetes Secret immutable attribute to DisCo

[FelixPhipps]

Closes VC-46158

[davidnoyes]

What manual validation has been performed to check that this solves the issue for the DisCo team?

[FelixPhipps]

Manual testing steps

1. Build the agent binary:
   go build -o _bin/ark ./cmd/ark

2. Create an immutable Secret Save as secret.yaml:

  kind: Secret
  metadata:
    name: tls-client-auth-only-rsa
    namespace: cyberark
  immutable: true
  type: kubernetes.io/tls
  data:
    tls.crt: dGVzdA==
    tls.key: dGVzdA==

4. Apply:
   kubectl apply -f secret.yaml

5. Prepare agent config including secrets gatherer Save as agent_config.yaml:

cluster_description: "manual test of immutable secret"
period: "12h0m0s"
data-gatherers:
  - kind: k8s-dynamic
    name: ark/secrets
    config:
      resource-type:
        version: v1
        resource: secrets
      field-selectors:
        - type!=kubernetes.io/dockercfg
        - type!=kubernetes.io/dockerconfigjson
        - type!=bootstrap.kubernetes.io/token
        - type!=helm.sh/release.v1

6. Run agent one-shot:

  --logging-format=json \
  --log-level=6 \
  --one-shot \
  --output-path out.txt \
  --agent-config-file agent_config.yaml

7. Verify immutable field is present in output out.txt:

[mladen-rusev-cyberark]

Tested manually via the testing steps with the 3 states of immutable (true, false, missing). Output is as expected.