Make store implementations pluggable

This for instance allows hydra-queue-runner to add the S3 backend
at runtime.

src/libstore/local-binary-cache-store.cc

@@ -41,4 +41,13 @@ std::string LocalBinaryCacheStore::getFile(const std::string & path)
     return readFile(binaryCacheDir + "/" + path);
 }
 
+static RegisterStoreImplementation regStore([](const std::string & uri) -> std::shared_ptr<Store> {
+    if (std::string(uri, 0, 7) != "file://") return 0;
+    auto store = std::make_shared<LocalBinaryCacheStore>(std::shared_ptr<Store>(0),
+        "", "", // FIXME: allow the signing key to be set
+        std::string(uri, 7));
+    store->init();
+    return store;
+});
+
 }