Keep private services reachable during control plane outages

[scotwells]

Summary

Private services can retain access during control plane outages. Persistent access remains active until the policy is withdrawn or its VPC lifetime changes. Services can also request time-bounded access. API validation requires one access lifetime policy.

Test plan

• CI accepts persistent access without a deadline.
• CI rejects persistent access with a deadline.
• CI rejects bounded access without a deadline.
• CI preserves VPC identity requirements for translated access.