Add UDP over IP socket concept

[apcountryman]

Add UDP over IP socket concept (::picolibrary::IP::UDP::Socket_Concept) and associated mock (::picolibrary::Testing::Automated::IP::UDP::Mock_Socket).

• [x] The Socket_Concept class should be defined in the include/picolibrary/ip/udp.h/source/picolibrary/ip/udp.cc header/source file pair
• [x] The Mock_Socket class should be defined in the include/picolibrary/testing/automated/ip/udp.h/source/picolibrary/testing/automated/ip/udp.cc header/source file pair
• [x] Sockets should be expected to support the following operations:
  • [x] Socket_Concept() noexcept;
  • [x] Socket_Concept( Socket_Concept && source ) noexcept;
  • [x] ~Socket_Concept() noexcept;
  • [x] auto operator=( Socket_Concept && expression ) noexcept -> Socket_Concept &;
  • [x] void bind( Endpoint const & endpoint = Endpoint{} ) noexcept;: Bind the socket to a local endpoint
  • [x] auto local_endpoint() const noexcept -> Endpoint;: Get the socket's local endpoint
  • [x] auto transmit( Endpoint const & endpoint, std::uint8_t const * begin, std::uint8_t const * end ) noexcept -> Result<void>;: Transmit a datagram to a remote endpoint
  • [x] auto receive( std::uint8_t * begin, std::uint8_t * end ) noexcept -> Result<Reception_Result>;: Receive a datagram from a remote endpoint
  • [x] void close() noexcept;: Close the socket
• [x] The Mock_Socket class should include a movable mock handle
• [x] Documentation