net: samples: sockets: http_get: Add secure socket offload support

Update the http_get sockets sample to enable association of secure
tags with certificate filenames.

This allows certificates and keys to be provisioned to the file
system on secure flash, without having to include the actual
certificates in the application.

Validated on the cc3220sf_launchxl board.

Signed-off-by: Gil Pitney <gil.pitney@linaro.org>