Userland: Introduce ConfigServer and LibConfig

ConfigServer is an IPC service that provides access to application
configuration and settings. The idea is to replace all uses of
Core::ConfigFile with IPC requests to ConfigServer.

This first cut of the API is pretty similar to Core::ConfigFile.

The old:

    auto config = Core::ConfigFile::open_for_app("App");
    auto value = config->read_entry("Group", "Key");

The new:

    auto value = Config::read_string("App", "Group", "Key");

ConfigServer uses the ~/.config directory as its backing store
and all the files remain human-editable. :^)

Userland/Libraries/LibConfig/Client.cpp

@@ -0,0 +1,50 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibConfig/Client.h>
+
+namespace Config {
+
+static RefPtr<Client> s_the = nullptr;
+
+Client& Client::the()
+{
+    if (!s_the || !s_the->is_open())
+        s_the = Client::construct();
+    return *s_the;
+}
+
+String Client::read_string(StringView domain, StringView group, StringView key, StringView fallback)
+{
+    return read_string_value(domain, group, key).value_or(fallback);
+}
+
+i32 Client::read_i32(StringView domain, StringView group, StringView key, i32 fallback)
+{
+    return read_i32_value(domain, group, key).value_or(fallback);
+}
+
+bool Client::read_bool(StringView domain, StringView group, StringView key, bool fallback)
+{
+    return read_bool_value(domain, group, key).value_or(fallback);
+}
+
+void Client::write_string(StringView domain, StringView group, StringView key, StringView value)
+{
+    async_write_string_value(domain, group, key, value);
+}
+
+void Client::write_i32(StringView domain, StringView group, StringView key, i32 value)
+{
+    async_write_i32_value(domain, group, key, value);
+}
+
+void Client::write_bool(StringView domain, StringView group, StringView key, bool value)
+{
+    async_write_bool_value(domain, group, key, value);
+}
+
+}