[aapt2] Add placeholder --feature-flags flag

Adding a placeholder so that soong build integration can happen in
parallel with the implementation of this flag.

Bug: 297373084
Bug: 306024510
Test: m aapt2
Change-Id: If9df57daaa871c11e146a8a8f8113fceb2fd96c2
diff --git a/tools/aapt2/cmd/Link.h b/tools/aapt2/cmd/Link.h
index 1b1e93bd..a08f385 100644
--- a/tools/aapt2/cmd/Link.h
+++ b/tools/aapt2/cmd/Link.h
@@ -323,6 +323,7 @@
             "should only be used together with the --static-lib flag.",
         &options_.merge_only);
     AddOptionalSwitch("-v", "Enables verbose logging.", &verbose_);
+    AddOptionalFlagList("--feature-flags", "Placeholder, to be implemented.", &feature_flags_args_);
   }
 
   int Action(const std::vector<std::string>& args) override;
@@ -347,6 +348,7 @@
   std::optional<std::string> stable_id_file_path_;
   std::vector<std::string> split_args_;
   std::optional<std::string> trace_folder_;
+  std::vector<std::string> feature_flags_args_;
 };
 
 }// namespace aapt