Implemented an input validation mechanism (#4)

This PR contains the work done to implement the `ValidateInputUseCase` use case and the `InputValidationRule` protocol, that is essential to define custom validation rules for inputs. In addition, the `NotEmptyValidationRule` and `NotNilValidationRule` rules have also been implemented.

Reviewed-on: #4
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

Tests/DiscogsService/Cases/Public/Middlewares/UserAgentMiddlewareTests.swift

@@ -0,0 +1,25 @@
+// ===----------------------------------------------------------------------===
+//
+// 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 DiscogsService
+import Testing
+
+@Suite("User Agent Middleware", .tags(.middleware))
+struct UserAgentMiddlewareTests {
+
+//    @Test func <#test function name#>() async throws {
+//        // Write your test here and use APIs like `#expect(...)` to check expected conditions.
+//    }
+
+}