[flutter_plugin_tools] Add a federated PR safety check (#4329)

Creates a new command to validate that PRs don't change platform interface packages and implementations at the same time, to try to prevent ecosystem-breaking changes. See https://github.com/flutter/flutter/issues/89518 for context.

Per the explanation in the issue, this has carve-outs for:
- Changes to platform interfaces that aren't published (allowing for past uses cases such as making a substantive change to an implementation, and making minor adjustments to comments in the PI package based on those changes).
- Things that look like bulk changes (e.g., a mass change to account for a new lint rule)

Fixes https://github.com/flutter/flutter/issues/89518

script/tool/lib/src/main.dart

@@ -13,6 +13,7 @@ import 'build_examples_command.dart';
 import 'common/core.dart';
 import 'create_all_plugins_app_command.dart';
 import 'drive_examples_command.dart';
+import 'federation_safety_check_command.dart';
 import 'firebase_test_lab_command.dart';
 import 'format_command.dart';
 import 'license_check_command.dart';
@@ -49,6 +50,7 @@ void main(List<String> args) {
     ..addCommand(BuildExamplesCommand(packagesDir))
     ..addCommand(CreateAllPluginsAppCommand(packagesDir))
     ..addCommand(DriveExamplesCommand(packagesDir))
+    ..addCommand(FederationSafetyCheckCommand(packagesDir))
     ..addCommand(FirebaseTestLabCommand(packagesDir))
     ..addCommand(FormatCommand(packagesDir))
     ..addCommand(LicenseCheckCommand(packagesDir))