Create the skeleton of the C++ -> Rust migration tool.

PiperOrigin-RevId: 448016954
diff --git a/common/file_io.h b/common/file_io.h
index 7ab9ff8..b2924d6 100644
--- a/common/file_io.h
+++ b/common/file_io.h
@@ -11,6 +11,8 @@
 
 namespace crubit {
 
+absl::StatusOr<std::string> GetFileContents(absl::string_view path);
+
 absl::Status SetFileContents(absl::string_view path,
                              absl::string_view contents);