[WIP] Export Private Key (trustwallet#345)

* add func for exporting private key

* change return type to Data

* add error case for exporting private key

Trust/EtherClient/EtherKeystore.swift

@@ -235,13 +235,26 @@ open class EtherKeystore: Keystore {
         guard let account = getAccount(for: account.address) else {
             return .failure(.accountNotFound)
         }
-
         do {
             let data = try keyStore.export(account: account, password: password, newPassword: newPassword)
             return (.success(data))
         } catch {
             return (.failure(.failedToDecryptKey))
         }
+
+    }
+
+    func exportPrivateKey(account: Account) -> Result<Data, KeystoreError> {
+        guard let password = getPassword(for: account) else {
+            return .failure(KeystoreError.accountNotFound)
+        }
+        do {
+            let privateKey = try keyStore.exportPrivateKey(account: account, password: password)
+            return .success(privateKey)
+        } catch {
+            return .failure(KeystoreError.failedToExportPrivateKey)
+        }
+
     }
 
     func delete(wallet: Wallet) -> Result<Void, KeystoreError> {

Trust/EtherClient/KeyStoreError.swift

@@ -14,6 +14,7 @@ enum KeystoreError: LocalizedError {
     case failedToParseJSON
     case accountNotFound
     case failedToSignMessage
+    case failedToExportPrivateKey
 
     var errorDescription: String? {
         switch self {
@@ -39,6 +40,8 @@ enum KeystoreError: LocalizedError {
             return "Account not found"
         case .failedToSignMessage:
             return "Failed to sign message"
+        case .failedToExportPrivateKey:
+            return "Failed to export private key"
         }
     }
 }