Third-party package json serialization support

[arnetheduck]

json_serialization includes support for a number of third-party packages, such as std/net and parts of chronos.

What stands out is that neither of these types are part of json_serialization per se, but these libraries also don't depend on json_serialization on their own.

Having a single writeValue/readValue for these types is necessary to prevent ambiguity in overload resolution, but it's an open question where they should live:

• in json_serialization: doesn't allow testing, nimble package does not depend on them (and rightly so) - also, which types/packages should be supported this way? there's no good rule.
• in json_serialization, with a feature flag: solves testing with a feature-gated dep flag, but still, why in json_ser at all?
• in the package - the reverse problem applies, ie why should chronos or stint provide json_serialization support for its types?
• in a separate repo/package - "correctest" solution but messy to use

This issue is opened to collect pros/cons of each solution and maybe find new ones - eventually, it should translate into a manual recommendation.