Signer infrastructure: Prep for #9076

This sets up infrastructure in libutil to allow for signing other than
by a secret key in memory. #9076 uses this to implement remote signing.

(Split from that PR to allow reviewing in smaller chunks.)

Co-Authored-By: Raito Bezarius <masterancpp@gmail.com>

src/libstore/keys.hh

@@ -0,0 +1,10 @@
+#pragma once
+///@file
+
+#include "signature/local-keys.hh"
+
+namespace nix {
+
+PublicKeys getDefaultPublicKeys();
+
+}