Define jsonUnknown trait

[benoitlouy]

This PR is meant to replace #135

[CLAassistant]

All committers have signed the CLA.

[benoitlouy]

I made the jsonUnknown trait member exclusive within a structure. I believe it makes sense only to allow a single member to collect unknown fields during json parsing.

[benoitlouy]

Should we update the jsonUnknown trait to only make it applicable to members targeting a map with members of type Document?
If allowing the trait to be applied on Document, someone could assign something else than a Document object to that member.

Edit: I made the update. I think it makes sense and makes writing JSON encoder more straightforward

[Baccata]

So, to answer your question, I'd like for this to be eventually extended to allow for annotating union members, to allow for capturing unknown values.

As long as we can amend the selector to also cater to that use-case in the future, I'm fine with it.

[benoitlouy]

Once we want to extend this trait to support unions, we can update the selector to
:is(structure > member :test(> map > member > document), union > member :test(> document))
This will allow the trait to be applied to a union member targeting a Document.

[Baccata]

@benoitlouy the code looks good to me. All that's left I think is adding documentation here to describe the expected behaviour associated to the trait when the protocols support it.

[Baccata]

@lewisjkl I'm happy with this. Feel free to review/merge/release