Add non-intrusive flavour of DERIVE_SERDE

[seingierf]

Hello!

Would it make sense to have a non-intrusive version of the DERIVE_SERDE macro? I usually need to serialize/deserialize structures from APIs which I would rather not modify. I guess I've become used to the non-intrusive macro that nlohmann json provides.

This is more a discussion than a PR since I am not familiar with the code yet ;-)

[injae]

I also think it would be nice to have that feature. 👍

[injae]

It seems there's an issue with CI not working due to the use of an outdated version of Ubuntu in the current GitHub Actions setup. After the merge of this #60 , it should be possible to proceed with the current task.