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>

Sources/MarvelService/Internal/Use Cases/GenerateHashUseCase.swift

@@ -0,0 +1,60 @@
+//===----------------------------------------------------------------------===
+//
+// 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 CryptoKit
+
+import struct Foundation.Data
+import struct Foundation.TimeInterval
+
+/// A use case that generates a MD5 hash value, which is required to authenticate any request.
+struct GenerateHashUseCase {
+    
+    // MARK: Properties
+    
+    /// A private key.
+    private let privateKey: String
+    
+    /// A public key.
+    private let publicKey: String
+
+    // MARK: Initializers
+    
+    /// Initializes this use case.
+    /// - Parameters:
+    ///   - privateKey: A private key.
+    ///   - publicKey: A public key.
+    init(
+        privateKey: String,
+        publicKey: String,
+    ) {
+        self.privateKey = privateKey
+        self.publicKey = publicKey
+    }
+    
+    // MARK: Functions
+    
+    /// Generates a MD5 hash value out of a given public key, private key and a timestamp.
+    /// - Parameter timestamp: A timestamp that changes on a request-by-request basis.
+    /// - Returns: A MD5 hash generated out of a private key, an public key, and a timestamp.
+    func callAsFunction(
+        timestamp: TimeInterval
+    ) -> String {
+        let stringToHash = timestamp.asString + self.privateKey + self.publicKey
+        let dataToHash = Data(stringToHash.utf8)
+
+        return Insecure.MD5
+            .hash(data: dataToHash)
+            .map { String(format: "%02x", $0) }
+            .joined()
+    }
+    
+}