Introduce a BuiltInBuildSystemAdapter that can be used to transition BuildSystem to a type that implements BSP

Sources/BuildSystemIntegration/BuildServerBuildSystem.swift

@@ -74,6 +74,12 @@ package actor BuildServerBuildSystem: MessageHandler {
     self.delegate = delegate
   }
 
+  package weak var messageHandler: BuiltInBuildSystemMessageHandler?
+
+  package func setMessageHandler(_ messageHandler: any BuiltInBuildSystemMessageHandler) {
+    self.messageHandler = messageHandler
+  }
+
   /// The build settings that have been received from the build server.
   private var buildSettings: [DocumentURI: FileBuildSettings] = [:]