Release version 0.12.8

Author: ddebowczyk

docs/release-notes/v0.12.8.md

@@ -1,4 +1,6 @@
+- composer.json cleanup
 - MessageRole now supports OpenAI 'developer' role, other drivers recognize and properly convert it
+- Removed missed dependency on experimental code in one of Structure traits
 - Moved tests into subpackage-specific directories
 - Added tests for EventDispatcher class
 - Added tests for Message class