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-binary-cache-store.cc

@@ -0,0 +1,14 @@
+#include <gtest/gtest.h>
+
+#include "local-binary-cache-store.hh"
+
+namespace nix {
+
+TEST(LocalBinaryCacheStore, constructConfig)
+{
+    LocalBinaryCacheStoreConfig config{"local", "/foo/bar/baz", {}};
+
+    EXPECT_EQ(config.binaryCacheDir, "/foo/bar/baz");
+}
+
+} // namespace nix