Refactor unix domain socket store config (#11109)

Following what is outlined in #10766 refactor the uds-remote-store such
that the member variables (state) don't live in the store itself but in
the config object.

Additionally, the config object includes a new necessary constructor
that takes a scheme & authority.

Tests are commented out because of linking errors with the current config system.
When there is a new config system we can reenable them.

Co-authored-by: John Ericson <John.Ericson@Obsidian.Systems>

src/libstore/local-fs-store.cc

@@ -8,6 +8,20 @@
 
 namespace nix {
 
+LocalFSStoreConfig::LocalFSStoreConfig(PathView rootDir, const Params & params)
+    : StoreConfig(params)
+    // Default `?root` from `rootDir` if non set
+    // FIXME don't duplicate description once we don't have root setting
+    , rootDir{
+        this,
+        !rootDir.empty() && params.count("root") == 0
+            ? (std::optional<Path>{rootDir})
+            : std::nullopt,
+        "root",
+        "Directory prefixed to all other paths."}
+{
+}
+
 LocalFSStore::LocalFSStore(const Params & params)
     : Store(params)
 {