UID2-7580: add graceful termination support to opt-out

[swibi-ttd]

Summary

• register the shared PodTerminationMonitor during opt-out startup with the existing 3-second default
• prepare a writable termination-marker directory in the non-root container image
• cover startup registration and configured interval behavior

Test plan

• mvn clean test (255 tests)
• build the Docker image
• verify the uid2-optout user can create /app/pod_terminating/pod_terminating
• publish snapshot image 4.10.5-alpha-234-SNAPSHOT
• update the uid2-deployment branch to use that snapshot in UID2 test
• deploy the branch to UID2 test through uid2-infra #1190
• after all test pods run the snapshot, terminate one pod and verify health becomes 503, the process remains alive during the 60-second drain, and no termination-attributable ALB 5xx occurs

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