Created S3StorageClient protocol and conformed S3 type to it

Sources/S3Storage/S3StorageClient.swift

@@ -0,0 +1,16 @@
+import S3
+
+/// The custom `S3Client` type required by `S3Storage`.
+public protocol S3StorageClient: S3Client {
+    
+    /// Gets a URLBuilder implementation instance from a container.
+    func urlBuilder(for container: Container) -> URLBuilder
+}
+
+extension S3: S3StorageClient {
+    
+    /// Gets a URLBuilder implementation instance from a container.
+    public func urlBuilder(for container: Container) -> URLBuilder {
+        return S3URLBuilder(container, defaultBucket: defaultBucket, config: signer.config)
+    }
+}