External Parent CA for Platform Certificates

[sanchezl]

Summary

• Extends the PKI configuration API to allow administrators to provide an external CA certificate and key that cross-signs all platform signer certificates
• Enables all internally generated certificates to chain to a customer's enterprise root CA
• Preserves OpenShift's existing internal trust model, automated rotation, and self-healing

Details

• Adds externalParentCA field to the existing PKIProfile in the PKI resource
• Cross-signing produces a second certificate for each signer (same key pair, signed by the external parent CA) alongside the existing self-signed certificate
• Cross-signed certs are appended to existing CA bundles for chain verification
• Supports both Day-1 (install-config.yaml) and Day-2 (PKI resource edit) provisioning
• Builds on the unified library-go cert creation code path from the configurable PKI enhancement

Tracking: https://issues.redhat.com/browse/OCPSTRAT-2029
Depends on: /enhancements/security/internal-pki-config.md

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