Add constructor from existing UDP socket

[alekseifedotov]

Problem

Sometimes users might want to start a new process in sandboxed environment without network access, for example from systemd PrivateNetwork or any other userspace sandboxing using network namespaces.

In such environment it is not possible to open a new UDP socket to datadog daemon, the only way to get connectivity outside of the sandbox is by using inherited file descriptors. For example from sd_listen_fds

Proposed solution

• Add a method to create Client from a UDP socket. The socket itself could be created from raw file descriptor.