feat(evm): add grant management for EVM wallets

useragent/lib/features/connection/evm.dart

@@ -0,0 +1,56 @@
+import 'package:arbiter/features/connection/connection.dart';
+import 'package:arbiter/proto/evm.pb.dart';
+import 'package:arbiter/proto/user_agent.pb.dart';
+import 'package:protobuf/well_known_types/google/protobuf/empty.pb.dart';
+
+Future<List<WalletEntry>> listEvmWallets(Connection connection) async {
+  await connection.send(UserAgentRequest(evmWalletList: Empty()));
+
+  final response = await connection.receive();
+  if (!response.hasEvmWalletList()) {
+    throw Exception(
+      'Expected EVM wallet list response, got ${response.whichPayload()}',
+    );
+  }
+
+  final result = response.evmWalletList;
+  switch (result.whichResult()) {
+    case WalletListResponse_Result.wallets:
+      return result.wallets.wallets.toList(growable: false);
+    case WalletListResponse_Result.error:
+      throw Exception(_describeEvmError(result.error));
+    case WalletListResponse_Result.notSet:
+      throw Exception('EVM wallet list response was empty.');
+  }
+}
+
+Future<void> createEvmWallet(Connection connection) async {
+  await connection.send(UserAgentRequest(evmWalletCreate: Empty()));
+
+  final response = await connection.receive();
+  if (!response.hasEvmWalletCreate()) {
+    throw Exception(
+      'Expected EVM wallet create response, got ${response.whichPayload()}',
+    );
+  }
+
+  final result = response.evmWalletCreate;
+  switch (result.whichResult()) {
+    case WalletCreateResponse_Result.wallet:
+      return;
+    case WalletCreateResponse_Result.error:
+      throw Exception(_describeEvmError(result.error));
+    case WalletCreateResponse_Result.notSet:
+      throw Exception('Wallet creation returned no result.');
+  }
+}
+
+String _describeEvmError(EvmError error) {
+  return switch (error) {
+    EvmError.EVM_ERROR_VAULT_SEALED =>
+      'The vault is sealed. Unseal it before using EVM wallets.',
+    EvmError.EVM_ERROR_INTERNAL || EvmError.EVM_ERROR_UNSPECIFIED =>
+      'The server failed to process the EVM request.',
+    _ => 'The server failed to process the EVM request.',
+  };
+}