Fix faulty json.Marshal behavior for embeds types.NetConf

[s1061123]

Fix #1096

[coveralls]

coverage: 63.69%. remained the same
when pulling 499596a on s1061123:fix/1096
into 73debca on containernetworking:main.

[LionelJouin]

This way the users would need to change types.NetConf with types.NetConfType: https://go.dev/play/p/j2wjAgA_s-K
The current way would still not work: https://go.dev/play/p/fHVOmTrH8p_K

[s1061123]

@LionelJouin Updated to avoid to change user side code.

[LionelJouin]

Looks good: https://go.dev/play/p/7ncmsg8-uL0

[s1061123]

@squeed PTAL?