feat(argo-rollouts): Allow templating in notification templates

[cktpw]

Checklist:

• I have bumped the chart version according to versioning
• I have updated the documentation according to documentation
• I have updated the chart changelog with all the changes that come with this pull request according to changelog.
• Any new values are backwards compatible and/or have sensible default.
• I have signed off all my commits as required by DCO.
• I have created a separate pull request for each chart according to pull requests
• My build is green (troubleshooting builds).

[yu-croco]

Any problem if we just support tpl ? As you can see there is no format parameters in all of argo-helm, though we have some logics to support tpl.

{{- with .Values.notifications.notifiers }}
  {{- tpl (toYaml .) $ | nindent 2 }}
{{- end }}

[cktpw]

Both Argo notifications and Helm use {{ and }} for interpolation. So if someone wants to use {{.rollout.metadata.namespace}} with tpl they'll have to escape the {{ and }} with like this: {{"{{"}}.rollout.metadata.namespace{{"}}"}}.

Using tpl alone would throw errors if someone just do {{.rollout.metadata.namespace}} because that doesn't exist in Helm's context.

The reason I had to do this is because rollouts doesn't support the context.rolloutsUrl like ArgoCD does with context.argocdUrl and I need to provide different Rollout's root URL per Rollout's deployment.

Here's how I did it with the enhancement above. With this method, I can provide a different value for notifications.rolloutsUrl for each deployment of Rollouts.

  notifications:
    secret:
      create: false

    rolloutsUrl: https://rollouts.staging.example.com

    notifiers:
      service.slack: |
        token: $slack-token

    triggers:
      trigger.on-rollout-updated: |
        - send: [rollout-updated]

    templates:
      format: template
      template.rollout-updated: |
        slack:
          attachments: |
            [{
              "title": "Rollout for {{"{{"}}.rollout.metadata.name{{"}}"}} in progress",
              "title_link": "{{.Values.notifications.rolloutsUrl}}/rollouts/rollout/{{"{{"}}.rollout.metadata.namespace{{"}}"}}/{{"{{"}}.rollout.metadata.name{{"}}"}}",
              "color": "#ffe000",
              "fields": [],
              "mrkdwn_in": ["fields"]
            }]