tpl support for summary field

[maximveksler]

What this PR does / why we need it:

Allow fetching values for summary field from Values / ConfigMap. Useful for automated flux rollout.

Example usage:

alertlist:
  - name: groundcover-incloud-version
    spec:
      summary: '{{ printf "%s - %s" (get (lookup "v1" "ConfigMap" "flux-system" "cluster-info").data "workspace") (get (lookup "v1" "ConfigMap" "flux-system" "cluster-info").data "region") | quote }}'
      providerRef:
        name: platform-updates
      eventSeverity: info
      eventSources:
        - kind: "HelmRelease"
          name: "groundcover-incloud"

Which issue this PR fixes

(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)

• fixes #

Special notes for your reviewer:

Checklist

• [x] DCO signed
• [x] Chart Version bumped
• [x] helm-docs are updated
• [x] Helm chart is tested
• [ ] Update artifacthub.io/changes in Chart.yaml
• [ ] Run make reviewable

[maximveksler]

@haarchri would appreciate feedback on this please.

[maximveksler]

@stefanprodan let's get this merged?