Git object hashing in libutil

This is the core functionality but just unit-tested and not yet made
part of the store layer. This is because there is some tech debt around
(a) repeated boilerplate hashing objects (b) better integration of the
new `SourceAccessor` type that needs to be cleaned up first.

Part of RFC 133

Co-Authored-By: Matthew Bauer <mjbauer95@gmail.com>
Co-Authored-By: Carlo Nucera <carlo.nucera@protonmail.com>
Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
Co-authored-by: Florian Klink <flokli@flokli.de>

src/libutil/experimental-features.cc

@@ -96,6 +96,14 @@ constexpr std::array<ExperimentalFeatureDetails, numXpFeatures> xpFeatureDetails
             [`nix`](@docroot@/command-ref/new-cli/nix.md) for details.
         )",
     },
+    {
+        .tag = Xp::GitHashing,
+        .name = "git-hashing",
+        .description = R"(
+            Allow creating (content-addressed) store objects which are hashed via Git's hashing algorithm.
+            These store objects will not be understandable by older versions of Nix.
+        )",
+    },
     {
         .tag = Xp::RecursiveNix,
         .name = "recursive-nix",