feat(useragent): initial impl

useragent/lib/features/pk_manager.dart

@@ -0,0 +1,19 @@
+
+import 'package:flutter/services.dart';
+
+enum KeyAlgorithm {
+  rsa, ecdsa, ed25519
+}
+
+// The API to handle without storing the private key in memory. 
+//The implementation will use platform-specific secure storage and signing capabilities.
+abstract class KeyHandle {
+  KeyAlgorithm get alg;
+  Future<List<int>> sign(List<int> data);
+  Future<List<int>> getPublicKey();
+}
+
+abstract class KeyManager {
+  Future<KeyHandle?> get();
+  Future<KeyHandle> create();
+}