Get length of packet from datagram sockets (UdpSocket, UnixDatagram)

[nnishant776]

Summary

Provide an API to peek/get the length of the datagram

The unstable feature unix_socket_peek adds methods peek and peek_from for UnixDatagram and UnixStream. While they allow to peek at the the data, however, the API returns the number of bytes read on success. To get the length of the datagram instead of the bytes read, in an API similar to peek, MSG_TRUNC ored with the MSG_PEEK flag can be passed to the recv call, which would only peek at the data but return the length the next datagram on success.

Motivation

This can be helpful in use cases for memory usage optimization where we can allocate buffers dynamically with just enough size for reading the data packet. This is helpful when the connections or message are infrequent, for example, an application configuration message.