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/Extensions/TimeInterval+Computed.swift

@@ -0,0 +1,25 @@
+//===----------------------------------------------------------------------===
+//
+// 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.TimeInterval
+
+extension TimeInterval {
+    
+    // MARK: Functions
+    
+    /// Converts a time interval to a string value.
+    /// - Returns: A time interval as a string.
+    var asString: String {
+        .init(format: "%f", self)
+    }
+    
+}