GH-1123 - Detect and warn about events configured for externalization published outside transaction context

[yunhobb]

Summary

Adds warning log when events configured for externalization are published outside a transactional context.

Fixes #1123

Problem

Events configured with @Externalized that are published outside @Transactional methods fail silently - they're not persisted to the registry and
externalization never occurs.

Solution

The PersistentApplicationEventMulticaster now detects this condition and logs a warning with clear guidance on how to fix the issue.

Changes

• Added EventExternalizationConfiguration dependency to PersistentApplicationEventMulticaster
• Implemented detection logic that checks: transaction active, has transactional listeners, event externalized
• Used ObjectProvider for optional dependency to maintain backward compatibility
• Updated tests