Create template migration snippets

[aarnq]

Proposed feature

When we update a larger set of components we most likely need a migration snippet to do some special steps for that stack.

So, we should add custom upgrade snippets that can be reused between versions, as many of those can be reused between versions.

Additionally, it would then be possible to add rollback support there, which then doesn't have to be reinvented between versions.

Proposed alternatives

One example is OpenSearch, as it has requirements for when things are upgraded so certain configuration jobs run after the core components have been updated.

Additional context

Note, that these migration snippets must never do unnecessary changes to a cluster!

Example, it should not assume it needs to always rerun opensearch-configurer because opensearch might have been upgraded, it should be clear conditions that reruns it when it has upgraded.

Definition of done

• Migration template includes predefined snippets
  • Fluentd
  • Grafana
  • Harbor
  • OpenSearch
  • Prometheus
  • Thanos
• This new feature is covered by contributor/migration documentation