samples: net: sockets: big_http_download: Update root certificate

The download server that the origin server redirects to seems now to
use different certificate (signed by a different root CA). Therefore
add the additional root CA to the certificate list and refactor the
sample code a bit to allow to easily extend/replace certificates in the
future.

Bump the mbed TLS heap size a bit to accommodate the extra registered
certificate.

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