Add `created-by` annotation to `ManagedControlPlane`

[maximiliantech]

Description

On the resources Project and Workspace there is a webhook implemented which will add core.openmcp.cloud/created-by annotation to the resource on creation.

The resource then looks like this:

apiVersion: core.openmcp.cloud/v1alpha1
kind: Project
metadata:
  annotations:
    core.openmcp.cloud/created-by: user@example.com

This is missing for the ManagedControlPlane resource.

Goal

A webhook should add the core.openmcp.cloud/created-by on the ManagedControlPlane resource during CREATION time as well.

Tip

By looking on the implementation in the project-workspace-operator, it is solved like so:
https://github.com/openmcp-project/project-workspace-operator/blob/f534796f239942c7686e3517ddcddee2757f3133/api/core/v1alpha1/common_webhook.go#L41-L49
which will get called here:
https://github.com/openmcp-project/project-workspace-operator/blob/f534796f239942c7686e3517ddcddee2757f3133/api/core/v1alpha1/project_webhook.go#L48

Tasks

• Implement webhook.CustomDefaulter interface
• ...

Acceptance Criteria

• Code has been reviewed by other team members
• New / changed code is documented
• Unit Tests created for new code or existing Unit Tests updated
• Integration Test Suite updated
• Enduser Documentation updated (if applicable)
• Successful demonstration in Review