samples: net: http_server: use PSA Crypto instead of legacy Mbed TLS APIs

This commit also moves the TLS (overlay) configuration to a separate file
named "overlay-tls.conf". In this way the simple HTTP (without TLS) will
not enable Mbed TLS.

A new test case is added to sample.yaml to test HTTPS scenario.

Signed-off-by: Valerio Setti <vsetti@baylibre.com>