feat: make RequestIdentifier header configurable via trace_header setting
- Add TraceHeaderConfig to router settings with configurable header_name
- Update RequestIdentifier middleware to accept String instead of &'static str
- Modify get_application_builder to use configurable trace header
- Add trace_header configuration to all relevant TOML files
- Set default header_name to 'x-request-id' for backward compatibility
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Co-Authored-By: Claude <noreply@anthropic.com>