plugin_manager 重新拆分，增加扩展

src/plugin_system/events/events.py

@@ -0,0 +1,14 @@
+from enum import Enum
+
+
+class EventType(Enum):
+    """
+    事件类型枚举类
+    """
+
+    ON_MESSAGE = "on_message"
+    ON_PLAN = "on_plan"
+    POST_LLM = "post_llm"
+    AFTER_LLM = "after_llm"
+    POST_SEND = "post_send"
+    AFTER_SEND = "after_send"