feat: improve local artifact proxy use

[felangel]

Description

Currently, in order to use a local instance of the artifact proxy, we need to manually change two places:

1. shorebird/third_party/flutter/bin/internal/shared.sh

   # Either clones or pulls the Shorebird Flutter repository, depending on whether FLUTTER_PATH exists.
   function update_flutter {
   if [[ -d "$FLUTTER_PATH" ]]; then
   git -C "$FLUTTER_PATH" fetch
   else
   git clone --filter=tree:0 https://github.com/shorebirdtech/flutter.git --no-checkout "$FLUTTER_PATH"
   fi
   # -c to avoid printing a warning about being in a detached head state.
   git -C "$FLUTTER_PATH" -c advice.detachedHead=false checkout "$FLUTTER_VERSION"
   SHOREBIRD_ENGINE_VERSION=`cat "$FLUTTER_PATH/bin/internal/engine.version"`
   echo "Shorebird Engine • revision $SHOREBIRD_ENGINE_VERSION"
   # Install Shorebird Flutter Artifacts
   - FLUTTER_STORAGE_BASE_URL=https://download.shorebird.dev $FLUTTER_PATH/bin/flutter --version
   + FLUTTER_STORAGE_BASE_URL=http://localhost:8080 $FLUTTER_PATH/bin/flutter --version  
   }

2. shorebird/packages/shorebird_cli/lib/src/shorebird_process.dart

   Map<String, String> _environmentOverrides({
   required String executable,
   }) {
   if (executable == 'flutter') {
     // If this ever changes we also need to update the `shorebird` shell
     // wrapper which downloads runs Flutter to fetch artifacts the first time.
   -     return {'FLUTTER_STORAGE_BASE_URL': 'https://download.shorebird.dev'};
   +     return {'FLUTTER_STORAGE_BASE_URL': 'http://localhost:8080'};
   }
   
   return {};
   }
   }

In addition, the local build.gradle needs to be modified to allow http:

allprojects {
    repositories {
        google()
        mavenCentral()

+       all {
+           allowInsecureProtocol = true
+       }
    }
}

It would be nice to expose an environment variable that can be specified to override this without modifying the source e.g. SHOREBIRD_ARTIFACT_PROXY_URL.

Not sure about how to improve the build.gradle experience but open to suggestions.