net: lib: sntp: add support for unspecified IP-family type

`sntp_simple()` was forcing to resolve SNTP-server's URL into IPv4 address
addresses. This was not allowing sntp_init() to succeed in case the system
 did not support IPv4 addresses (returning EPFNOSUPPORT, ie. Protocol
Family error).
Now by default SNTP has unspecified family type and it relies on
`net_getaddrinfo_addr_str()` to be able to resolve literal server URLs
into the supported IP family type.

Signed-off-by: Marco Argiolas <marco.argiolas@ftpsolutions.com.au>