Move ParseSink to its own header

We will soon add a new implemenation so the one for NARs in `archive.cc`
isn't the only one.

Co-Authored-By: Matthew Bauer <mjbauer95@gmail.com>
Co-Authored-By: Carlo Nucera <carlo.nucera@protonmail.com>

src/libutil/fs-sink.hh

@@ -0,0 +1,25 @@
+#pragma once
+///@file
+
+#include "types.hh"
+#include "serialise.hh"
+
+namespace nix {
+
+/**
+ * \todo Fix this API, it sucks.
+ */
+struct ParseSink
+{
+    virtual void createDirectory(const Path & path) { };
+
+    virtual void createRegularFile(const Path & path) { };
+    virtual void closeRegularFile() { };
+    virtual void isExecutable() { };
+    virtual void preallocateContents(uint64_t size) { };
+    virtual void receiveContents(std::string_view data) { };
+
+    virtual void createSymlink(const Path & path, const std::string & target) { };
+};
+
+}