Add UDP over IP socket support to IP network stack concept

[apcountryman]

Add UDP over IP socket support to IP network stack concept (::picolibrary::IP::Network_Stack_Concept).

• IP network stacks should be expected to provide the following members types:
  • UDP_Socket: The type of UDP socket that is used to interact with the network stack
• IP network stacks should be expected to support the following operations:
  • auto make_udp_socket() noexcept -> UDP_Socket;: Construct a UDP socket
• Documentation