Add --logging-format to cache-server and use ISO8601 timestamps in JSON logs

[xrstf]

Summary

• Adds --logging-format flag support to cache-server
• Implements custom JSON logger that uses ISO8601 timestamps instead of UNIX timestamps
• Replaces Kubernetes upstream JSON logger imports with kcp-specific implementation across all commands

Changes

• cache-server: Added logging options configuration and early initialization of logging format
• New package pkg/logging/json: Custom JSON logger factory that wraps upstream implementation but uses ISO8601 time encoding
• Import updates: All commands now import github.com/kcp-dev/kcp/pkg/logging/json/register instead of k8s.io/component-base/logs/json/register

Test plan

• Build and run cache-server with --logging-format=json flag
• Verify JSON log output contains ISO8601 formatted timestamps (e.g., "ts":"2026-03-13T10:30:45.123Z") instead of UNIX timestamps
• Verify all other kcp commands continue to work with the updated imports

* Change "ts" in JSON logging format to be ISO 8601 instead of UNIX timestamps.
* Add `--logging-format` flag to the cache-server.

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[xrstf]

/kind cleanup

[xrstf]

/retest

[xrstf]

/retest

[xrstf]

/retest