samples: net: big_http_download: Add minimal support for redirect

It's often the case with TLS file servers, that the download link does
not lead to downloaded file, but rather redirects to another server.
Therefore it makes sense to have a minimal support in the sample for
HTTP redirection, so that the file can still be downloaded.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>