FileSystem: Add a Custody class that represents a parent/child guardianship.

A custody is kind of a directory entry abstraction that represents a single
entry in a parent directory that tells us the name of a child inode.

The idea here is for path resolution to produce a chain of custody objects.

Kernel/FileSystem/Custody.h

@@ -0,0 +1,32 @@
+#pragma once
+
+#include <AK/AKString.h>
+#include <AK/RetainPtr.h>
+#include <AK/Retainable.h>
+
+class Inode;
+
+class Custody : public Retainable<Custody> {
+public:
+    static Retained<Custody> create(Custody* parent, const String& name, Inode& inode)
+    {
+        return adopt(*new Custody(parent, name, inode));
+    }
+
+    ~Custody();
+
+    Custody* parent() { return m_parent.ptr(); }
+    const Custody* parent() const { return m_parent.ptr(); }
+
+    Inode& inode() { return *m_inode; }
+    const Inode& inode() const { return *m_inode; }
+
+    const String& name() const { return m_name; }
+
+private:
+    Custody(Custody* parent, const String& name, Inode&);
+
+    RetainPtr<Custody> m_parent;
+    String m_name;
+    Retained<Inode> m_inode;
+};