samples: sockets: Add big_http_download sample

This sample downloads a more or less big file (~6MB as preconfigured)
over HTTP and checks its hash for integrity. It also repeat such a
download indefinitely, counting total number of bytes transferred.

This is thus a kind of traffic load testing sample. (Ported to C
from MicroPython original).

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>