Registry Cache: describe technical agreements and decisions in ADR and verify it with POC

[tobiscr]

Description

We agreed with Jellies on following technical approach for detecing changes related to the registry cache configuration:

1. Frogs will provide a dedicated CRD on SKR side which allows customers to specify their individual registry cache configuration (and maybe more -tbc)
2. Watcher can be used to detected changes applies on this CRD. It will report change-events to a dedicated service on KCP side
3. The service on KCP side has to be provided by Frogs (e.g. dedicated controller in KIM). The controller has two responsibilities:
   1. Retrieve events
   2. Check in time-intervals each SKR for changes applied on our CRD
   3. Ensure Shoot will be updated. 2 Options:
      1. option 1: notify KIM about it (e..g by using Add possibility to trigger / suspend a Shoot reconciliation by KIM #605 ) and KIM will reconcile the cluster and retrieve the configuration from SKR and introduce it to Shoot-Spec
      2. option 2: apply the change on Shoot-Spec

AC:

• Create ADR, evaluating pros/cons of the flow and proposal which solution should be used
  • Share results in Framefrog team
• Prepare POC for the solution to ensure it's technical feasibility and find potential pitfalls / risks upfront

Reasons

Support detection of changes on SKR side which require a reconciliation of Shoot-Specs.

Attachments

[tobiscr]

This approach could be adapted also for #135