api: stub out SignImage endpoint

BUG=b:296089271
TEST=./run_tests api/controller/image_unittest.py::SignImageTest
TEST=./api/contrib/call_scripts/image__sign_image

Change-Id: Ic71b9b66572431b975a75e2980c49bdb1a48e925
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/4828447
Reviewed-by: Benjamin Shai <bshai@google.com>
Reviewed-by: Alex Klein <saklein@chromium.org>
Commit-Queue: Jack Neus <jackneus@google.com>
Tested-by: Jack Neus <jackneus@google.com>
diff --git a/api/controller/image.py b/api/controller/image.py
index 4f0a895..969623d 100644
--- a/api/controller/image.py
+++ b/api/controller/image.py
@@ -655,3 +655,23 @@
             for uri in uris:
                 _output_proto.instructions.add().instructions_file_path = uri
     return controller.RETURN_CODE_SUCCESS
+
+
+@faux.all_empty
+@validate.validation_complete
+def SignImage(
+    _input_proto: "image_pb2.SignImageRequest",
+    _output_proto: "image_pb2.SignImageResponse",
+    _config: "api.config.ApiConfig",
+):
+    """Sign artifacts based on the given config.
+
+    Args:
+        input_proto: Input proto.
+        _output_proto: Output proto.
+        config: The API call config.
+
+    Returns:
+        A controller return code (e.g. controller.RETURN_CODE_SUCCESS).
+    """
+    return controller.RETURN_CODE_SUCCESS