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/types.py

@@ -1,4 +1,7 @@
-from sqlalchemy import CHAR, TypeDecorator
+import enum
+from typing import Generic, TypeVar
+
+from sqlalchemy import CHAR, VARCHAR, TypeDecorator
 from sqlalchemy.dialects.postgresql import UUID
 
 
@@ -24,3 +27,51 @@ class StringUUID(TypeDecorator):
         if value is None:
             return value
         return str(value)
+
+
+_E = TypeVar("_E", bound=enum.StrEnum)
+
+
+class EnumText(TypeDecorator, Generic[_E]):
+    impl = VARCHAR
+    cache_ok = True
+
+    _length: int
+    _enum_class: type[_E]
+
+    def __init__(self, enum_class: type[_E], length: int | None = None):
+        self._enum_class = enum_class
+        max_enum_value_len = max(len(e.value) for e in enum_class)
+        if length is not None:
+            if length < max_enum_value_len:
+                raise ValueError("length should be greater than enum value length.")
+            self._length = length
+        else:
+            # leave some rooms for future longer enum values.
+            self._length = max(max_enum_value_len, 20)
+
+    def process_bind_param(self, value: _E | str | None, dialect):
+        if value is None:
+            return value
+        if isinstance(value, self._enum_class):
+            return value.value
+        elif isinstance(value, str):
+            self._enum_class(value)
+            return value
+        else:
+            raise TypeError(f"expected str or {self._enum_class}, got {type(value)}")
+
+    def load_dialect_impl(self, dialect):
+        return dialect.type_descriptor(VARCHAR(self._length))
+
+    def process_result_value(self, value, dialect) -> _E | None:
+        if value is None:
+            return value
+        if not isinstance(value, str):
+            raise TypeError(f"expected str, got {type(value)}")
+        return self._enum_class(value)
+
+    def compare_values(self, x, y):
+        if x is None or y is None:
+            return x is y
+        return x == y