Make embassy_net Config const-constructable

[kleinesfilmroellchen]

While the configuration functions like dhcpv4() are const, since the structs involved are non-exhaustive, and Default cannot be used in const contexts (like any trait method), they cannot be constructed in a const context. This precludes the configuration from being put into ROM/Flash memory via static or const, which could save quite a bit of space especially considering that many devices have more ROM than RAM available.

All that is needed here is a default-like method on all the affected configuration structs that is const.