aconfig: add project scaffolding

Introduce a new tool, aconfig, to manage build time configurations, such
as feature flags.

This CL adds a project skeleton. Later CLs will add the actual
implementation.

Note: there is no need for an explicit TEST_MAPPING file;
rust_test_host are automatically included in host-unit-tests and
explicitly adding these tests via TEST_MAPPING will cause an error.

Bug: 279485059
Test: m aconfig && aconfig
Test: atest aconfig.test
Change-Id: I94047a19a55ae4d45b4fd7a0c8105a91ddcdfd79
diff --git a/tools/aconfig/protos/aconfig.proto b/tools/aconfig/protos/aconfig.proto
new file mode 100644
index 0000000..989c398
--- /dev/null
+++ b/tools/aconfig/protos/aconfig.proto
@@ -0,0 +1,23 @@
+// Copyright (C) 2023 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License
+
+// Placeholder proto file. Will be replaced by actual contents.
+
+syntax = "proto3";
+
+package android.aconfig;
+
+message Placeholder {
+  string name = 1;
+}