build(hook): wire APT package action callbacks

Install the APT configuration fragment used to invoke the snapshot
helper before dpkg processes package files.

Request protocol version 3 and a dedicated information file descriptor so
the helper can classify package installations, upgrades, downgrades, and
removals without attempting to infer actions from the apt command line.

90apt-zfs-snapshot.conf

@@ -0,0 +1,7 @@
+# APT sends the package action list to the helper before invoking dpkg.
+DPkg::Tools::options::/usr/local/sbin/apt-zfs-snapshot::Version "3";
+DPkg::Tools::options::/usr/local/sbin/apt-zfs-snapshot::InfoFD "3";
+DPkg::Pre-Install-Pkgs
+{
+    "/usr/local/sbin/apt-zfs-snapshot";
+};