[native_assets_cli] AssetDownloader

I'm punting this from `package:native_assets_cli`.

This should likely live in a separate helper package.

```dart
// Copyright (c) 2024, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'dart:io';

import 'package:logging/logging.dart';

import '../../native_assets_cli.dart';

abstract class Builder {
Future run({
required BuildConfig config,
required BuildOutput output,
required Logger? logger,
});
}

// TODO(dacoharkes): Should we really add this? It seems too specific.
// E.g. what about varying file names, zipped downloads etc. etc. ?
class AssetDownloader implements Builder {
final Uri Function(OS, Architecture) downloadUri;

/// Asset identifier.
///
/// If omitted, no asset will be added to the build output.
final String assetName;

AssetDownloader({
required this.downloadUri,
required this.assetName,
});

@override
Future run({
required BuildConfig config,
required BuildOutput output,
required Logger? logger,
}) async {
Uri? targetUri;
if (!config.dryRun) {
final downloadUri2 = downloadUri(
config.targetOS,
config.targetArchitecture!,
);
final fileName =
downloadUri2.pathSegments.lastWhere((element) => element.isNotEmpty);
targetUri = config.outputDirectory.resolve(fileName);
final request = await HttpClient().getUrl(downloadUri2);
final response = await request.close();
await response.pipe(File.fromUri(targetUri).openWrite());
}

output.addAsset(
CCodeAsset(
package: config.packageName,
name: assetName,
file: targetUri,
linkMode: LinkMode.dynamic,
dynamicLoading: BundledDylib(),
os: config.targetOS,
architecture: config.targetArchitecture,
),
);
}
}
```

It might implement the shared interface `Builder`:

* https://github.com/dart-lang/native/issues/995