Added UnreferencedStringEnumeration marker type for enumerations referenced by the spec

Author: dimitribouniol

Sources/WebAuthn/Helpers/UnreferencedStringEnumeration.swift

@@ -0,0 +1,33 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the WebAuthn Swift open source project
+//
+// Copyright (c) 2022 the WebAuthn Swift project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of WebAuthn Swift project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+/// An enumeration type that is not referenced by other parts of the Web IDL because that would preclude other values from being used without updating the specification and its implementations.
+/// - SeeAlso: [WebAuthn Level 3 Editor's Draft §2.1.1. Enumerations as DOMString types](https://w3c.github.io/webauthn/#sct-domstring-backwards-compatibility)
+public protocol UnreferencedStringEnumeration: RawRepresentable, Codable, ExpressibleByStringLiteral, Hashable, Comparable where RawValue == String {
+    init(_ rawValue: RawValue)
+}
+
+extension UnreferencedStringEnumeration {
+    public init(rawValue: RawValue) {
+        self.init(rawValue)
+    }
+    
+    public init(stringLiteral value: String) {
+        self.init(value)
+    }
+    
+    public static func < (lhs: Self, rhs: Self) -> Bool {
+        lhs.rawValue < rhs.rawValue
+    }
+}