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.h

@@ -0,0 +1,71 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <ConfigServer/ConfigClientEndpoint.h>
+#include <ConfigServer/ConfigServerEndpoint.h>
+#include <LibCore/File.h>
+#include <LibCore/Promise.h>
+#include <LibCore/StandardPaths.h>
+#include <LibIPC/ServerConnection.h>
+
+namespace Config {
+
+class Client final
+    : public IPC::ServerConnection<ConfigClientEndpoint, ConfigServerEndpoint>
+    , public ConfigClientEndpoint {
+    C_OBJECT(Client);
+
+public:
+    String read_string(StringView domain, StringView group, StringView key, StringView fallback);
+    i32 read_i32(StringView domain, StringView group, StringView key, i32 fallback);
+    bool read_bool(StringView domain, StringView group, StringView key, bool fallback);
+
+    void write_string(StringView domain, StringView group, StringView key, StringView value);
+    void write_i32(StringView domain, StringView group, StringView key, i32 value);
+    void write_bool(StringView domain, StringView group, StringView key, bool value);
+
+    static Client& the();
+
+private:
+    explicit Client()
+        : IPC::ServerConnection<ConfigClientEndpoint, ConfigServerEndpoint>(*this, "/tmp/portal/config")
+    {
+    }
+};
+
+inline String read_string(StringView domain, StringView group, StringView key, StringView fallback = {})
+{
+    return Client::the().read_string(domain, group, key, fallback);
+}
+
+inline i32 read_i32(StringView domain, StringView group, StringView key, i32 fallback = 0)
+{
+    return Client::the().read_i32(domain, group, key, fallback);
+}
+
+inline bool read_bool(StringView domain, StringView group, StringView key, bool fallback = false)
+{
+    return Client::the().read_bool(domain, group, key, fallback);
+}
+
+inline void write_string(StringView domain, StringView group, StringView key, StringView value)
+{
+    Client::the().write_string(domain, group, key, value);
+}
+
+inline void write_i32(StringView domain, StringView group, StringView key, i32 value)
+{
+    Client::the().write_i32(domain, group, key, value);
+}
+
+inline void write_bool(StringView domain, StringView group, StringView key, bool value)
+{
+    Client::the().write_bool(domain, group, key, value);
+}
+
+}