Paypal: Pending

Start TransactionService, preliminary support for `pending_ext` transaction state.
```ruby
def create_transaction_response(transaction, gateway_fields = {})
  TransactionResponse.call({
    transaction: transaction,
    gateway_fields: gateway_fields
  })
end

# Call site:
create_transaction_response(transaction, create_preauth_paypal_fields(:multi-currency))
```