Add grafana.initContainers for Airgapped Analytics

[marcleblanc2]

Adding grafana.initContainers, so we can create a native sidecar using the aws-cli Docker image for Airgapped Analytics to work for customers using AWS RDS with IRSA authentication, without the Grafana pod crashing and rebooting twice before stabilizing. Follow-up to #924.

Checklist

• Follow the manual testing process
• Update changelog
• Update Kubernetes update doc — not needed, no user-facing upgrade steps

Test plan

Tested with values:

grafana:
  serviceAccount:
    create: false
    name: grafana-irsa
  initContainers:
    - name: rds-token
      restartPolicy: Always
      image: public.ecr.aws/aws-cli/aws-cli:2.36.27
      startupProbe:
        exec:
          command: ["test", "-s", "/run/rds-token/password"]
        periodSeconds: 1
        failureThreshold: 60
      # ... writes /run/rds-token/password in a loop

• helm template renders rds-token under spec.template.spec.initContainers with restartPolicy: Always, grafana alone under containers, and serviceAccountName: grafana-irsa
• kubectl create --dry-run=server accepts the rendered StatefulSet on EKS 1.36
• Default values (initContainers: []) render no initContainers key, so existing deployments are unchanged
• helm unittest charts/sourcegraph: 137 passed; helm lint: clean
• Verified the Grafana pod starts up without crashing