Kernel/SysFS: Add exposing interface for DisplayConnectors

Under normal conditions (when mounting SysFS in /sys), there will be a
new directory in the /sys/devices directory called "graphics".
For now, under that directory there will be only a sub-directory called
"connectors" which will contain all DisplayConnectors' details, each in
its own sub-directory too, distinguished in naming with its minor
number.

Therefore, /sys/devices/graphics/connectors/MINOR_NUMBER/ will contain:
- General device attributes such as mutable_mode_setting_capable,
  double_buffering_capable, flush_support, partial_flush_support and
  refresh_rate_support. These values are exposed in the ioctl interface
  of the DisplayConnector class too, but these can be useful later on
  for command line utilities that want/need to expose these basic
  settings.
- The EDID blob, simply named "edid". This will help userspace to fetch
  the edid without the need of using the ioctl interface later on.

Kernel/FileSystem/SysFS/Subsystems/DeviceIdentifiers/CharacterDevicesDirectory.h

@@ -13,7 +13,10 @@
 namespace Kernel {
 
 class Device;
+class DisplayConnector;
 class SysFSCharacterDevicesDirectory final : public SysFSDirectory {
+    friend class DisplayConnector;
+
 public:
     virtual StringView name() const override { return "char"sv; }
     static NonnullRefPtr<SysFSCharacterDevicesDirectory> must_create(SysFSDeviceIdentifiersDirectory const&);