Implemented the Auth middleware (#3)

This PR contains the work done to implement the `AuthMiddleware` middleware that, when plugged into a `Client` object, adds authentication parameters to any request the client would make.

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

Tests/MarvelService/Types/Extensions/String+Samples.swift

@@ -0,0 +1,21 @@
+//===----------------------------------------------------------------------===
+//
+// 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 Marvel API key samples.
+    enum Key {
+        /// A Marvel API private key sample.
+        static let `private` = "SomePrivateKey"
+        /// A Marvel API public key sample.
+        static let `public` = "SomePublicKey"
+    }
+}

Tests/MarvelService/Types/Extensions/Tag+Customs.swift

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+//===----------------------------------------------------------------------===
+//
+// 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
+//
+//===----------------------------------------------------------------------===
+
+import Testing
+
+extension Tag {
+    
+    // MARK: Constants
+    
+    /// A flag that indicates tests for a type extension.
+    @Tag static var `extension`: Self
+    
+    /// A flag that indicates tests for a middleware type.
+    @Tag static var middleware: Self
+    
+    /// A flag that indicates tests for a use case type.
+    @Tag static var useCase: Self
+}

Tests/MarvelService/Types/Namespaces/Input.swift

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+//===----------------------------------------------------------------------===
+//
+// 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
+//
+//===----------------------------------------------------------------------===
+
+import struct Foundation.Date
+import struct Foundation.TimeInterval
+
+/// A namespace assigned for input arguments on test cases.
+enum Input {
+    
+    // MARK: Constants
+
+    /// A list of timestamps samples.
+    static let timestamps: [TimeInterval] = [
+        Date(timeIntervalSince1970: 0).timeIntervalSince1970,
+        Date(timeIntervalSince1970: 1_000).timeIntervalSince1970,
+        Date(timeIntervalSince1970: 1_000_000).timeIntervalSince1970,
+        Date(timeIntervalSince1970: 1_000_000_000).timeIntervalSince1970,
+    ]
+}

Tests/MarvelService/Types/Namespaces/Output.swift

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+//===----------------------------------------------------------------------===
+//
+// 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
+//
+//===----------------------------------------------------------------------===
+
+/// A namespace assigned for output arguments on test cases, that are expected results.
+enum Output {}