Extend <netElement> with <designator> child element

Description: The topology element shall be extended with a repeatable optional child element .

Links:

• Minutes ETCS use case working group meeting 18.10.2021: [https://www.railml.org/en/event-reader/railml-is-etcs-telco-2021-10-18.html]
• Forum:
  • Christian Rahmig, 17.01.2022: https://www.railml.org/forum/index.php?t=msg&goto=2879&#msg_2879
  • Karl-Friedemann Jerosch, 16.03.2022: https://www.railml.org/forum/index.php?t=msg&th=848&goto=2960&#msg_2960
• Wiki documentation:
  • IS:netElement: https://wiki3.railml.org/wiki/IS:netElement