feat: add artifact_proxy server (#222)

packages/artifact_proxy/README.md

@@ -0,0 +1,26 @@
+# Artifact Proxy
+
+This is a tool for proxying Flutter artifacts from a derived Flutter engine
+revision back to the base Flutter engine revision. This is useful for
+when you need to modify _some_ of the Flutter artifacts but not all of them.
+
+This is a development tool which map requests to Google
+Storage (either Shorebird's bucket or the official Flutter buckets).
+
+## Usage
+
+Uses `config.yaml` to configure the engine revisions and artifact overrides.
+
+```bash
+# Run locally with hot-reload enabled.
+DEV=true dart --enable-vm-service run bin/server.dart
+```
+
+And then in a separate terminal:
+
+```
+FLUTTER_STORAGE_BASE_URL=http://localhost:8080 flutter precache -a
+```
+
+You should use a separate checkout of Flutter when running this, so you don't
+poison the cache of your main Flutter checkout.