Preserve user-defined order of pre-shared certificates

[petar-savov]

The controller alphabetically sorts all certificates in the pre-shared-certs annotation, which contradicts GCP's documented behavior

For pre-shared certificates listed in the ingress.gcp.kubernetes.io/pre-shared-cert annotation, the order in which you list the certificates determines the primary certificate. The primary certificate, which is presented when no other certificate matches the client's request, is the first certificate listed in the annotation.

This prevents the user from controlling which certificate is primary. The same contradicting behavior was also documented in #kubernetes/ingress-gce#1309.

This solution

• preserves user-defined order of pre-shared certificates (as per GCP documentation)
• sorts managed certificates alphabetically (as per GCP documentation)
• maintains deduplication logic

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