Draft: cdma: allow IPv6 for generic AT+PPP connections

The generic dial_cdma_ready function forces AT+PPP connections to be IPv4-only, even if the modem advertises IPv6 support in its +CGDCONT response (via PDP type "IPV6" or "IPV4V6"). Allow both IPv4 and IPv6 to be configured via PPP.

For modems that do not advertise IPv6 support, an ipv6_config won't be created in dial_3gpp_ready, and thus IPv6 will not be activated.