Add the MountedSSHStore experimental feature

It will be implemented in the subsequent commits of this PR.

src/libutil/experimental-features.cc

@@ -262,6 +262,13 @@ constexpr std::array<ExperimentalFeatureDetails, numXpFeatures> xpFeatureDetails
             Allow the use of the [impure-env](@docroot@/command-ref/conf-file.md#conf-impure-env) setting.
         )",
     },
+    {
+        .tag = Xp::MountedSSHStore,
+        .name = "mounted-ssh-store",
+        .description = R"(
+            Allow the use of the [`mounted SSH store`](@docroot@/command-ref/new-cli/nix3-help-stores.html#experimental-ssh-store-with-filesytem-mounted).
+        )",
+    },
     {
         .tag = Xp::VerifiedFetches,
         .name = "verified-fetches",