Establish Trust Onboarding API <-> Provider from `ManagedControlPlane`

Description
As a platform admin I want to bring the new Onboarding API Provider to my umbrella ManagedControlPlane
When Crossplane Provider Onbaording API is enabled in an MCP, we want to automate its trust to the Onboarding API so that end users can order additional MCPs/WS/etc right away.

End user specifies provider in ManagedControlPlane

apiVersion: core.openmcp.cloud/v1alpha1
kind: ManagedControlPlane
metadata:
  name: yo-mcp
spec:
...
  components:
...
    crossplane:
      version: 1.18.0
      providers:
        - name: onboarding-api
          version: 1.0.0

End user creates a GitHub Service Issue with following info:

• a
  (@reshnm )

Technical steps

• Provider authenticates via ServiceAcocunt
• Provider creates Token, rotates regularly automated, with right audience

Next Steps

• Enable trust from MCP to Onboarding API
• v1: Partly manual process (Useres reach out via Github Support issue, process described in manual process)

Out of scope

• Automated trust enablement

Done Criteria

• End users can activate provider and Trust between MCP and Onboarding API is accomplished on their behalf
• Provider is visible in CRD browser and release channel
• End user guide exists
• Stretch: Provider can be activated in managed control plane