feat(api): Introduce WorkflowDraftVariable Model (#19737)

- Introduce `WorkflowDraftVariable` model and the corresponding migration.
- Implement `EnumText`,  a custom column type for SQLAlchemy designed
  to work seamlessly with enumeration classes based on `StrEnum`.

api/models/enums.py

@@ -14,3 +14,10 @@ class UserFrom(StrEnum):
 class WorkflowRunTriggeredFrom(StrEnum):
     DEBUGGING = "debugging"
     APP_RUN = "app-run"
+
+
+class DraftVariableType(StrEnum):
+    # node means that the correspond variable
+    NODE = "node"
+    SYS = "sys"
+    CONVERSATION = "conversation"