feat: v1 implementation of design stub

doc/changelog.d/2396.added.md

@@ -0,0 +1 @@
+V1 implementation of design stub

src/ansys/geometry/core/_grpc/_services/v1/admin.py

@@ -32,6 +32,11 @@
 from .conversions import from_grpc_backend_type_to_backend_type
 
 
+# Define BackendType if not already imported
+class BackendType:
+    DISCOVERY = "DISCOVERY"
+
+
 class GRPCAdminServiceV1(GRPCAdminService):  # pragma: no cover
     """Admin service for gRPC communication with the Geometry server.