feat(api): Introduce a VariableLoader interface.

The `VariableLoader` interface is used to load referenced variables
when running a single node.

api/core/workflow/variable_loader.py

@@ -0,0 +1,38 @@
+import abc
+from typing import Protocol
+
+from core.variables import Variable
+
+
+class VariableLoader(Protocol):
+    """Interface for loading variables based on selectors.
+
+    A `VariableLoader` is responsible for retrieving additional variables required during the execution
+    of a single node, which are not provided as user inputs.
+
+    NOTE(QuantumGhost): Typically, all variables loaded by a `VariableLoader` should belong to the same
+    application and share the same `app_id`. However, this interface does not enforce that constraint,
+    and the `app_id` parameter is intentionally omitted from `load_variables` to achieve separation of
+    concern and allow for flexible implementations.
+
+    Implementations of `VariableLoader` should almost always have an `app_id` parameter in
+    their constructor.
+
+    TODO(QuantumGhost): this is a temporally workaround. If we can move the creation of node instance into
+    `WorkflowService.single_step_run`, we may get rid of this interface.
+    """
+
+    @abc.abstractmethod
+    def load_variables(self, selectors: list[list[str]]) -> list[Variable]:
+        """Load variables based on the provided selectors. If the selectors are empty,
+        this method should return an empty list.
+
+        The order of the returned variables is not guaranteed. If the caller wants to ensure
+        a specific order, they should sort the returned list themselves.
+
+        :param: selectors: a list of string list, each inner list should have at least two elements:
+            - the first element is the node ID,
+            - the second element is the variable name.
+        :return: a list of Variable objects that match the provided selectors.
+        """
+        pass