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>

Sources/DiscogsService/Internal/Use Cases/ValidateInputUseCase.swift

@@ -0,0 +1,53 @@
+// ===----------------------------------------------------------------------===
+//
+// 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
+//
+// ===----------------------------------------------------------------------===
+
+/// A use case that validates an input against a set of validation rules.
+struct ValidateInputUseCase {
+    
+    // MARK: Properties
+    
+    /// A list of validation rules to match an input against.
+    private let rules: [any InputValidationRule]
+    
+    // MARK: Initializers
+    
+    /// Initializes this use case.
+    /// - Parameter rules: A list of validation rules to match an input against.
+    init(rules: any InputValidationRule...) {
+        self.rules = rules
+    }
+    
+    // MARK: Functions
+    
+#if swift(>=6.0)
+    /// Validates an input against a set of validation rules.
+    /// - Parameter input: An input to be validated against a set of rules, if any.
+    /// - Throws: An error of type ``InputValidationError`` in case an input failed any validation.
+    func callAsFunction(_ input: String?) throws(InputValidationError) {
+        for rule in rules {
+            try rule.validate(input)
+        }
+    }
+#else
+    /// Validates an input against a set of validation rules.
+    /// - Parameter input: An input to be validated against a set of rules, if any.
+    /// - Throws: An error of type ``InputValidationError`` in case an input failed any validation.
+    func callAsFunction(_ input: String?) throws {
+        for rule in rules {
+            try rule.validate(input)
+        }
+    }
+#endif
+
+}