Allow swapping UnifiedPush distributor

Fixes #120

lib/controllers/unified_push_distributors.dart

@@ -0,0 +1,26 @@
+import "dart:io";
+
+import "package:fast_immutable_collections/fast_immutable_collections.dart";
+import "package:flutter_riverpod/flutter_riverpod.dart";
+import "package:unifiedpush/unifiedpush.dart";
+
+class UnifiedPushDistributorsController extends AsyncNotifier<IList<String>> {
+  @override
+  Future<IList<String>> build() async {
+    if (!Platform.isLinux && !Platform.isAndroid) return .new();
+
+    return IList(
+      await UnifiedPush.getDistributors().timeout(
+        .new(seconds: 10),
+        onTimeout: () =>
+            throw Exception("UnifiedPush distributor check timed out."),
+      ),
+    );
+  }
+
+  static final provider =
+      AsyncNotifierProvider.autoDispose<
+        UnifiedPushDistributorsController,
+        IList<String>
+      >(UnifiedPushDistributorsController.new);
+}