Implemented the Authentication middleware (#3)

This PR contains the work done to implement the `AuthMiddleware` middleware, to authenticate the requests sent to the backend service, based on [their specifications](https://www.discogs.com/developers/#page:authentication).

In addition, some documentation has been added/updated and some boilerplate source code has been removed from the project.

Reviewed-on: #3
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

Sources/DiscogsService/Internal/Extensions/String+Constants.swift

@@ -0,0 +1,36 @@
+// ===----------------------------------------------------------------------===
+//
+// This source file is part of the DiscogsService open source project
+//
+// Copyright (c) 2025 Röck+Cöde VoF. and the DiscogsService project authors
+// Licensed under Apache license v2.0
+//
+// See LICENSE for license information
+// See CONTRIBUTORS for the list of DiscogsService project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+// ===----------------------------------------------------------------------===
+
+extension String {
+    /// An empty string.
+    static let empty = ""
+    
+    /// A namespaces assigned for the names of parameters.
+    enum Parameter {
+        /// A name for the consumer key.
+        static let key = "key"
+        /// A name for the consumer secret.
+        static let secret = "secret"
+        /// A name for the user token.
+        static let token = "token"
+    }
+    /// A namespaces assigned for the formats of string values.
+    enum Format {
+        /// A format for the consumer authentication header.
+        static let authConsumer = "Discogs \(String.Parameter.key)=%@, \(String.Parameter.secret)=%@"
+        /// A format for the user authentication header.
+        static let authUser = "Discogs \(String.Parameter.token)=%@"
+    }
+}
+