network: dhcp6pd: set default metric 256 for delegated prefix

network: dhcp6pd: set default metric 256 for delegated prefix

When Assign= in [DHCPv6PrefixDelegation] is enabled, then the kernel
will create the prefix route for the assigned address with metric 256.
When Assign= is disabled, then the kernel will create the route with
metric 1024.

For the default value, we should choose a smaller value (higher priority)
than 1024, as the unreachable routes for delegated prefix will be
configured with 1024.