Kernel/USB: Fetch configuration descriptors on enumeration

This also introduces a new class, `USBConfiguration` that stores a
configuration. The device, when instructed, sets this configuration and
holds a pointer to it so we have a record of what configuration is
currently active.

Kernel/Bus/USB/USBConfiguration.h

@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2022, Jesse Buhagiar <jesse.buhagiar@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Vector.h>
+#include <Kernel/Bus/USB/USBDescriptors.h>
+#include <Kernel/Bus/USB/USBDevice.h>
+
+namespace Kernel::USB {
+
+class Device;
+
+class USBConfiguration {
+public:
+    USBConfiguration() = delete;
+    USBConfiguration(Device& device, USBConfigurationDescriptor const configuration_descriptor)
+        : m_device(device)
+        , m_descriptor(configuration_descriptor)
+    {
+    }
+
+    Device const& device() const { return m_device; }
+
+    u8 interface_count() const { return m_descriptor.number_of_interfaces; }
+    u8 configuration_id() const { return m_descriptor.configuration_value; }
+    u8 attributes() const { return m_descriptor.attributes_bitmap; }
+    u16 max_power_ma() const { return m_descriptor.max_power_in_ma * 2u; } // Note: "Power" is used incorrectly here, however it's what it's called in the descriptor/documentation
+
+private:
+    Device& m_device;                  // Reference to the device linked to this configuration
+    USBConfigurationDescriptor m_descriptor; // Descriptor that backs this configuration
+};
+
+}