Added support for an API key to the AuthMiddleware middleware in the library target. (#4)

This PR contains the work done to extend the implementation of the `AuthMiddleware` middleware to handle an API key as well as private and public keys.

Reviewed-on: #4
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

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

@@ -0,0 +1,23 @@
+//===----------------------------------------------------------------------===
+//
+// This source file is part of the MarvelService open source project
+//
+// Copyright (c) -2025 Röck+Cöde VoF. and the MarvelService project authors
+// Licensed under the EUPL 1.2 or later.
+//
+// See LICENSE for license information
+// See CONTRIBUTORS for the list of MarvelService project authors
+//
+//===----------------------------------------------------------------------===
+
+extension String {
+    /// A namespace assigned for parameter keys of URI paths.
+    enum Parameter {
+        /// A Marvel API key parameter.
+        static let apiKey = "apikey"
+        /// A MD5 hash parameter.
+        static let hash = "hash"
+        /// A timestamp parameter.
+        static let timestamp = "ts"
+    }
+}