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/core/variables/consts.py

@@ -0,0 +1,7 @@
+# The minimal selector length for valid variables.
+#
+# The first element of the selector is the node id, and the second element is the variable name.
+#
+# If the selector length is more than 2, the remaining parts are the keys / indexes paths used
+# to extract part of the variable value.
+MIN_SELECTORS_LENGTH = 2

api/core/variables/utils.py

@@ -0,0 +1,8 @@
+from collections.abc import Iterable, Sequence
+
+
+def to_selector(node_id: str, name: str, paths: Iterable[str] = ()) -> Sequence[str]:
+    selectors = [node_id, name]
+    if paths:
+        selectors.extend(paths)
+    return selectors