[native_assets] Preparation existing tests for future of other (i.e. non-Code) assets (#160436)

In the future a hook may be invoked multiple times with different
`supportedAssetTypes` (soon to be renamed to `buildAssetTypes`).

The hook should only emit those asset types that are in
`supportedAssetTypes` - anything else is an error. Right now flutter
happens to invoke hooks only with `Code` asset types, but more asset
types are coming, for which this PR is a preparation for.

dev/integration_tests/link_hook/hook/build.dart

@@ -9,6 +9,10 @@ import 'package:native_toolchain_c/native_toolchain_c.dart';
 
 void main(List<String> args) async {
   await build(args, (BuildConfig config, BuildOutputBuilder output) async {
+    if (!config.supportedAssetTypes.contains(CodeAsset.type)) {
+      return;
+    }
+
     final String assetName;
     if (config.linkingEnabled) {
       // The link hook will be run. So emit an asset with a name that is

dev/integration_tests/link_hook/hook/link.dart

@@ -6,6 +6,9 @@ import 'package:native_assets_cli/code_assets.dart';
 
 void main(List<String> args) async {
   await link(args, (LinkConfig config, LinkOutputBuilder output) async {
+    if (!config.supportedAssetTypes.contains(CodeAsset.type)) {
+      return;
+    }
     final CodeAsset asset = config.codeAssets.single;
     final String packageName = config.packageName;
     output.codeAssets.add(