Add helm chart for NGTS-capable agent

[SgtCoDFish]

This has been tested to the point of being able to upload data to the backend through NGTS. I haven't been able to fully test that the uploaded data appears in the UI, but the upload itself is successful.

This doesn't add a release mechanism for the new chart, so adding this is still a no-op in terms of customer-facing functionality.

Example config:

config:
  tsgID: "1111111111"
  serverURL: https://1111111111.data.plane.example.com
  clusterName: test-cluster
  clusterDescription: "test cluster for agent"

volumes:
- name: custom-ca-volume
  configMap:
    name: custom-ca

volumeMounts:
- name: custom-ca-volume
  mountPath: /etc/ssl/certs

Example credentials:

apiVersion: v1
data:
  clientID: MmNkNzdlNzQtMmU3Ni0xMWYxLWIzN2QtYjIzZTk5MmMzYjAz
  privatekey.pem: <redacted>
kind: Secret
metadata:
  creationTimestamp: "2026-04-09T13:48:54Z"
  name: discovery-agent-credentials
  namespace: paloalto
  resourceVersion: "766"
  uid: f7aaca15-7bd4-4bb6-90fb-7d9d75501de5
type: Opaque

[SgtCoDFish]

I had a huddle with Georgi and walked him through the changes here. I explained that most of this is additive, so there's no impact from merging now. We can tweak afterwards - but we're merging this now.