Two new invoice related functions added and several breaking changes

Added

• Payee API function: get_payee_invoices_changed_since_timestamp
• Payer API function: get_payee_invoices_list_details

Changed

• Many function return values were changed:
  • Error maps are structs now. e.g. %{error_code: "...", error_message: "..."} is now %Tipalti.ClientError{error_code: "...", error_message: "..."}
  • A new Tipalti.RequestError struct will be returned for HTTP request errors
  • Payee.get_extended_payee_details_list/1 now returns {:ok, [Tipalti.PayeeExtended.t()]}
  • Payee.get_payee_details/1 now returns {:ok, Tipalti.Payee.t()}
  • Payee.payee_payable/2 now returns {:ok, true} or {:ok, false, reason}
  • Payee.payee_payment_method/1 now returns {:ok, String.t()}
  • Any function that used to return {:ok, :ok} now just returns :ok
  • Payer.create_or_update_invoices now returns the list of responses directly instead of wrapped in a map
  • refer to the documentation for any additional details