net: http_server: Enable v4-to-v6 mapping by default

Fixing the regression caused by 39498738 ("Allow service to be
created with NULL host"). If the host parameter is null when creating
the HTTP service, the IPv6 socket is created by default. This can cause
issues if both IPv4 and IPv6 are enabled, like in HTTP server sample,
and the HTTP client connection is done by IPv4.
To fix this, we need to enable IPv4-to-IPv6 mapping in order to allow
IPv6 socket to serve a IPv4 connection. Allow also user to override this
if needed.

Fixes #78112

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>