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/Cases/Internal/Extensions/TimeInterval+ComputedTests.swift

@@ -0,0 +1,90 @@
+//===----------------------------------------------------------------------===
+//
+// 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
+
+import struct Foundation.TimeInterval
+
+@testable import MarvelService
+
+@Suite("Time Interval computed properties", .tags(.extension))
+struct TimeIntervalComputedTests {
+    
+    // MARK: Properties tests
+    
+#if swift(>=6.2)
+    @Test(arguments: zip(
+        Input.timestamps,
+        Output.timestampAsString
+    ))
+    func `asString`(
+        timestamp: TimeInterval,
+        expects string: String
+    ) {
+        assertAsString(
+            timestamp: timestamp,
+            expects: string
+        )
+    }
+#else
+    @Test("asString", arguments: zip(
+        Input.timestamps,
+        Output.timestampAsString
+    ))
+    func asString(
+        timestamp: TimeInterval,
+        expects string: String
+    ) {
+        assertAsString(
+            timestamp: timestamp,
+            expects: string
+        )
+    }
+#endif
+    
+}
+
+// MARK: - Assertions
+
+private extension TimeIntervalComputedTests {
+    
+    // MARK: Functions
+    
+    /// Asserts the timestamp to string conversion.
+    /// - Parameters:
+    ///   - timestamp: A timestamp to convert.
+    ///   - string: An expected timestamp converted to string.
+    func assertAsString(
+        timestamp: TimeInterval,
+        expects string: String
+    ) {
+        // GIVEN
+        // WHEN
+        let result = timestamp.asString
+        
+        // THEN
+        #expect(result == string)
+    }
+
+}
+
+// MARK: - Constants
+
+private extension Output {
+    /// A list of outcomes that are expected from converting timestamps to string.
+    static let timestampAsString: [String] = [
+        "0.000000",
+        "1000.000000",
+        "1000000.000000",
+        "1000000000.000000"
+    ]
+}

Tests/MarvelService/Cases/Internal/Use Cases/GenerateHashUseCaseTests.swift

@@ -0,0 +1,95 @@
+//===----------------------------------------------------------------------===
+//
+// 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
+
+import struct Foundation.TimeInterval
+
+@testable import struct MarvelService.GenerateHashUseCase
+
+@Suite("Generate Hash Use Case", .tags(.useCase))
+struct GenerateHashUseCaseTests {
+    
+    // MARK: Functions
+    
+#if swift(>=6.2)
+    @Test(arguments: zip(
+        Input.timestamps,
+        Output.generatedHashes
+    ))
+    func `hash`(
+        timestamp: TimeInterval,
+        expects hash: String
+    ) async throws {
+        assertHash(
+            timestamp: timestamp,
+            expects: hash
+        )
+    }
+#else
+    @Test("hash", arguments: zip(
+        Input.timestamps,
+        Output.generatedHashes
+    ))
+    func hash(
+        timestamp: TimeInterval,
+        expects hash: String
+    ) async throws {
+        assertHash(
+            timestamp: timestamp,
+            expects: hash
+        )
+    }
+#endif
+    
+}
+
+// MARK: - Assertions
+
+private extension GenerateHashUseCaseTests {
+   
+    // MARK: Functions
+    
+    /// Asserts the MD5 hash generated from the use case.
+    /// - Parameters:
+    ///   - timestamp: A timestamp to use in the hash generation.
+    ///   - hash: An expected MD5 hash string as a result of the use case.
+    func assertHash(
+        timestamp: TimeInterval,
+        expects hash: String
+    ) {
+        // GIVEN
+        let useCase: GenerateHashUseCase = .init(
+            privateKey: .Key.private,
+            publicKey: .Key.public
+        )
+
+        // WHEN
+        let result = useCase(timestamp: timestamp)
+
+        // THEN
+        #expect(result == hash)
+    }
+    
+}
+
+// MARK: - Constants
+
+private extension Output {
+    /// A list of outcomes that are expected from the hash generation.
+    static let generatedHashes: [String] = [
+        "ef9ca6f930e56fb4f8a109a9003580fe",
+        "b500748e9f0aabc67ffc640ae9b87695",
+        "b537f18579112902b7ce046dddad558a",
+        "00fec88a254d42e3a439d49e14cd60d1"
+    ]
+}