Implemented the User Agent middleware (#6)

This PR contains the work done to implement the `UserAgentMiddleware` middleware that includes user agent information into a header of the requests sent by the `Client` type, as defined in the [Discogs documentation](https://www.discogs.com/developers/#page:home,header:home-general-information). For this purpose, the `CamelCaseValidationRule`, `SemanticVersionValidationRule` and `URLValidationRule` types were implemented and integrated into the existing `ValidateInputUseCase` type.

Reviewed-on: #6
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

Tests/DiscogsService/Types/Extensions/HTTPRequests+Inits.swift

@@ -0,0 +1,41 @@
+// ===----------------------------------------------------------------------===
+//
+// This source file is part of the DiscogsService open source project
+//
+// Copyright (c) 2025 Röck+Cöde VoF. and the DiscogsService project authors
+// Licensed under Apache license v2.0
+//
+// See LICENSE for license information
+// See CONTRIBUTORS for the list of DiscogsService project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+// ===----------------------------------------------------------------------===
+
+import struct HTTPTypes.HTTPFields
+import struct HTTPTypes.HTTPRequest
+
+extension HTTPRequest {
+    
+    // MARK: Initializers
+    
+    /// Initializes a HTTP request conveniently.
+    /// - Parameters:
+    ///   - method: A request method.
+    ///   - path: A value of the “:path” pseudo header field.
+    ///   - headerFields: A dictionary of request header fields.
+    init(
+        method: HTTPRequest.Method = .get,
+        path: String?,
+        headerFields: HTTPFields = [:]
+    ) {
+        self.init(
+            method: method,
+            scheme: nil,
+            authority: nil,
+            path: path,
+            headerFields: headerFields
+        )
+    }
+    
+}

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

@@ -18,6 +18,9 @@ extension Tag {
     
     // MARK: Constants
     
+    /// A tag that indicates tests for a type extension.
+    @Tag static var `extension`: Self
+    
     /// A tag that indicates tests for a middleware type.
     @Tag static var middleware: Self
     

Tests/DiscogsService/Types/Samples/String+Samples.swift

@@ -0,0 +1,21 @@
+// ===----------------------------------------------------------------------===
+//
+// This source file is part of the DiscogsService open source project
+//
+// Copyright (c) 2025 Röck+Cöde VoF. and the DiscogsService project authors
+// Licensed under Apache license v2.0
+//
+// See LICENSE for license information
+// See CONTRIBUTORS for the list of DiscogsService project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+// ===----------------------------------------------------------------------===
+
+extension String {
+    /// A namespace assigned for string samples on test cases.
+    enum Sample {
+        /// An operation ID sample.
+        static let operationId = "SomeOperationId"
+    }
+}

Tests/DiscogsService/Types/Samples/URL+Samples.swift

@@ -0,0 +1,23 @@
+// ===----------------------------------------------------------------------===
+//
+// This source file is part of the DiscogsService open source project
+//
+// Copyright (c) 2025 Röck+Cöde VoF. and the DiscogsService project authors
+// Licensed under Apache license v2.0
+//
+// See LICENSE for license information
+// See CONTRIBUTORS for the list of DiscogsService project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+// ===----------------------------------------------------------------------===
+
+import Foundation
+
+extension URL {
+    /// A namespace assigned for URL samples on test cases.
+    enum Sample {
+        /// A base URL sample.
+        static let baseURL = URL(string: "https://sample.domain.com")!
+    }
+}