Prepare system extension support

SystemExtension/Info.plist

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>NetworkExtension</key>
+	<dict>
+		<key>NEMachServiceName</key>
+		<string>group.io.nekohasekai.sfa.system</string>
+		<key>NEProviderClasses</key>
+		<dict>
+			<key>com.apple.networkextension.packet-tunnel</key>
+			<string>$(PRODUCT_MODULE_NAME).PacketTunnelProvider</string>
+		</dict>
+	</dict>
+</dict>
+</plist>

SystemExtension/PacketTunnelProvider.swift

@@ -0,0 +1,15 @@
+import Library
+import NetworkExtension
+
+class PacketTunnelProvider: ExtensionProvider {
+    override func startTunnel(options: [String: NSObject]?) async throws {
+        guard let usernameObject = options?["username"] else {
+            writeFatalError("missing start options")
+            return
+        }
+        let username = usernameObject as! NSString
+        FilePath.sharedDirectory = URL(filePath: "/Users/\(username)/Library/Group Containers/\(FilePath.groupName)")
+        self.username = String(username)
+        try await super.startTunnel(options: options)
+    }
+}

SystemExtension/SystemExtension.entitlements

@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>com.apple.developer.networking.networkextension</key>
+	<array>
+		<string>packet-tunnel-provider-systemextension</string>
+	</array>
+	<key>com.apple.security.application-groups</key>
+	<array>
+		<string>group.io.nekohasekai.sfa</string>
+	</array>
+</dict>
+</plist>

SystemExtension/main.swift

@@ -0,0 +1,11 @@
+import Foundation
+import Library
+import NetworkExtension
+
+Variant.useSystemExtension = true
+
+autoreleasepool {
+    NEProvider.startSystemExtensionMode()
+}
+
+dispatchMain()