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>

tests/unit/libstore/local-overlay-store.cc

@@ -0,0 +1,34 @@
+// FIXME: Odd failures for templates that are causing the PR to break
+// for now with discussion with @Ericson2314 to comment out.
+#if 0
+#  include <gtest/gtest.h>
+
+#  include "local-overlay-store.hh"
+
+namespace nix {
+
+TEST(LocalOverlayStore, constructConfig_rootQueryParam)
+{
+    LocalOverlayStoreConfig config{
+        "local-overlay",
+        "",
+        {
+            {
+                "root",
+                "/foo/bar",
+            },
+        },
+    };
+
+    EXPECT_EQ(config.rootDir.get(), std::optional{"/foo/bar"});
+}
+
+TEST(LocalOverlayStore, constructConfig_rootPath)
+{
+    LocalOverlayStoreConfig config{"local-overlay", "/foo/bar", {}};
+
+    EXPECT_EQ(config.rootDir.get(), std::optional{"/foo/bar"});
+}
+
+} // namespace nix
+#endif