Defined the InputValidationRule protocol in the library target.

Sources/DiscogsService/Internal/Errors/InputValidationError.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
+//
+// ===----------------------------------------------------------------------===
+
+/// A representation of all the possible validation error that could be thrown while validating an input.
+enum InputValidationError: Error {
+    /// An input is empty.
+    case inputIsEmpty
+    /// An input is nil.
+    case inputIsNil
+}