Kernel: Add KBufferBuilder, similar to StringBuilder but for KBuffer

This class works by eagerly allocating 1MB of virtual memory but only
adding physical pages on demand. In other words, when you append to it,
its memory usage will increase by 1 page whenever you append across a
page boundary (4KB.)

Kernel/KBufferBuilder.h

@@ -0,0 +1,28 @@
+#pragma once
+
+#include <AK/AKString.h>
+#include <Kernel/KBuffer.h>
+#include <stdarg.h>
+
+class KBufferBuilder {
+public:
+    using OutputType = KBuffer;
+
+    explicit KBufferBuilder();
+    ~KBufferBuilder() {}
+
+    void append(const StringView&);
+    void append(char);
+    void append(const char*, int);
+    void appendf(const char*, ...);
+    void appendvf(const char*, va_list);
+
+    KBuffer build();
+
+private:
+    bool can_append(size_t) const;
+    u8* insertion_ptr() { return m_buffer.data() + m_size; }
+
+    KBuffer m_buffer;
+    size_t m_size { 0 };
+};