Experiment with serving shared preferences over grpc

The idea is that for GitJournal's web version, we can have it talking
with the mobile version which runs a grpc server. All of the git + fs
logic will still run in the mobile and the web version will only have
the UI.

lib/experiments/grpc_server.dart

@@ -0,0 +1,96 @@
+import 'package:fixnum/fixnum.dart';
+import 'package:grpc/grpc.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+import 'package:gitjournal/generated/shared_preferences.pb.dart';
+import 'package:gitjournal/generated/shared_preferences.pbgrpc.dart';
+
+class SharedPreferencesService extends SharedPreferencesServiceBase {
+  final SharedPreferences pref;
+
+  SharedPreferencesService(this.pref);
+
+  @override
+  Future<BoolMessage> containsKey(ServiceCall _, StringMessage message) async {
+    return BoolMessage()..value = pref.containsKey(message.value);
+  }
+
+  @override
+  Future<StringListMessage> getKeys(ServiceCall _, EmptyMessage __) async {
+    return StringListMessage(value: pref.getKeys());
+  }
+
+  @override
+  Future<OptionalBool> getBool(ServiceCall _, StringMessage key) async {
+    return OptionalBool(value: pref.getBool(key.value));
+  }
+
+  @override
+  Future<OptionalInt> getInt(ServiceCall _, StringMessage key) async {
+    var ret = pref.getInt(key.value);
+    if (ret != null) {
+      return OptionalInt(value: Int64(ret));
+    }
+    return OptionalInt();
+  }
+
+  @override
+  Future<OptionalDouble> getDouble(ServiceCall _, StringMessage key) async {
+    return OptionalDouble(value: pref.getDouble(key.value));
+  }
+
+  @override
+  Future<OptionalString> getString(ServiceCall _, StringMessage key) async {
+    return OptionalString(value: pref.getString(key.value));
+  }
+
+  @override
+  Future<StringListMessage> getStringList(
+      ServiceCall _, StringMessage key) async {
+    return StringListMessage(value: pref.getStringList(key.value));
+  }
+
+  @override
+  Future<BoolMessage> setBool(ServiceCall _, SetBoolRequest req) async {
+    return BoolMessage(value: await pref.setBool(req.key, req.value));
+  }
+
+  @override
+  Future<BoolMessage> setInt(ServiceCall _, SetIntRequest req) async {
+    return BoolMessage(value: await pref.setInt(req.key, req.value.toInt()));
+  }
+
+  @override
+  Future<BoolMessage> setDouble(ServiceCall _, SetDoubleRequest req) async {
+    return BoolMessage(value: await pref.setDouble(req.key, req.value));
+  }
+
+  @override
+  Future<BoolMessage> setString(ServiceCall _, SetStringRequest req) async {
+    return BoolMessage(value: await pref.setString(req.key, req.value));
+  }
+
+  @override
+  Future<BoolMessage> setStringList(
+      ServiceCall _, SetStringListRequest req) async {
+    return BoolMessage(value: await pref.setStringList(req.key, req.value));
+  }
+
+  @override
+  Future<BoolMessage> remove(ServiceCall _, StringMessage key) async {
+    return BoolMessage(value: await pref.remove(key.value));
+  }
+}
+
+Future<void> main(List<String> args) async {
+  var pref = await SharedPreferences.getInstance();
+  final server = Server(
+    [SharedPreferencesService(pref)],
+    const <Interceptor>[],
+    CodecRegistry(codecs: const [GzipCodec(), IdentityCodec()]),
+  );
+  await server.serve(port: 50051);
+  print('Server listening on port ${server.port}...');
+}
+
+// todo: Create some kind of QR code for getting the URL