Add UDP over IP socket echo interactive test helper

[apcountryman]

Add UDP over IP socket echo interactive test helper (::picolibrary::Testing::Interactive::IP::UDP::echo()).

• [x] The echo() function should be defined in the include/picolibrary/testing/interactive/ip/udp.h/source/picolibrary/testing/interactive/ip/udp.cc header/source file pair
• [x] The echo() function should have the following template parameters:
  • [x] typename Network_Stack: The type of network stack to use to construct a socket
  • [x] typename Socket_Options_Configurator: A unary functor that takes the socket whose socket options are to be configured by reference, and configures the socket's socket options
• [x] The echo() function should have the following signature:

  template<typename Network_Stack, typename Socket_Options_Configurator>
  [[noreturn]] void echo( Reliable_Output_Stream & stream, Network_Stack & network_stack, Socket_Options_Configurator configure_socket_options, ::picolibrary::IP::UDP::Endpoint const & local_endpoint );

• [x] Documentation