Add --log-timestamp-utc flag for UTC log timestamps

[frederik12321]

Summary

Closes #3130

• Adds a --log-timestamp-utc CLI flag that switches text log timestamps from local time to UTC
• After the tracing migration (PR Tracing cleanup #7168), text logs default to local time (Local::now()), which resolved the original complaint about forced UTC
• This flag gives operators who prefer UTC (for slot comparison against tools like slots.symphonious.net, or multi-timezone debugging) an explicit opt-in
• JSON log timestamps remain UTC (Utc::now() with RFC3339 format) regardless of this flag

Implementation

Threads a log_timestamp_utc: bool field through LoggerConfig → init_tracing() → LoggingLayer. When the flag is set, on_event() uses Utc::now() instead of Local::now() for the text format timestamp.

SSE and libp2p/discv5 logging layers are left unchanged (local time) as they serve the GUI and internal debugging respectively.

Test plan

• cargo check passes with zero errors/warnings
• cargo fmt --all produces no diffs
• make lint (clippy) passes with zero warnings
• Manual: run lighthouse bn --log-timestamp-utc and verify timestamps show UTC
• Manual: run without flag and verify timestamps show local time (existing behavior)

🤖 Generated with Claude Code

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[frederik12321]

Closing this PR — same reason as #8913 and #8914.