Add an Immutable Plugin protocol (#4)

* Add ab Immutable Plugin protocol

* Add missing immutable keyPath

* Clean up usage and make test for immutable plugin

Sources/Plugin/ImmutablePlugin.swift

@@ -0,0 +1,24 @@
+public protocol ImmutablePlugin: Plugin where Output == Void {
+    /// The input value that the plugin will handle.
+    associatedtype Value
+
+    /// Handles the input value.
+    ///
+    /// - Parameters:
+    ///   - value: The input value that the plugin will handle.
+    /// - Throws: Any errors that occur during handling of the input value.
+    func handle(value: Value) async throws
+}
+
+extension ImmutablePlugin where Value == Input {
+    /// A keypath that provides access to the `immutable` property of the `Source`.
+    public var keyPath: WritableKeyPath<Source, ()> {
+        get { \.immutable }
+        set { }
+    }
+
+    public func handle(value: Value, output: inout ()) async throws {
+        try await handle(value: value)
+    }
+}
+

Tests/PluginTests/PluginTests.swift

@@ -86,4 +86,42 @@ final class PluginTests: XCTestCase {
         let token = try XCTUnwrap(urlRequest.request.allHTTPHeaderFields)["auth"]
         XCTAssertEqual(token, "token")
     }
+    
+    func testImmutablePlugin() async throws {
+        class MockService: Pluginable {
+            var plugins: [any Plugin] = []
+        }
+        
+        class CountPlugin: ImmutablePlugin {
+            typealias Source = MockService
+            
+            static let shared = CountPlugin()
+            
+            var count: Int = 0
+            
+            private init() {}
+            
+            func handle(value: Void) async throws {
+                count += 1
+            }
+        }
+        
+        let service = MockService()
+        
+        XCTAssertEqual(service.pluginTypes, [])
+        
+        service.register(
+            plugin: CountPlugin.shared
+        )
+        
+        XCTAssertEqual(service.pluginTypes, ["(input: (), output: ())"])
+        
+        try await service.handle(value: "Woot")
+        
+        XCTAssertEqual(CountPlugin.shared.count, 0)
+        
+        try await service.handle()
+        
+        XCTAssertEqual(CountPlugin.shared.count, 1)
+    }
 }