Define `#as_json` for selective serialisation

[tassja]

Implement #as_json or expand Clear JSON Deserialise model to accept annotation as macro arg in

• Clear (Allow passing in annotation arg in macro column place-labs/clear#7)

examples

• https://github.com/place-labs/staff-api/blob/c099e3befe407b8a2c6c8a3958d704706c19fa4b/src/models/tenant.cr#L67-L74

• https://github.com/place-labs/staff-api/blob/c099e3befe407b8a2c6c8a3958d704706c19fa4b/src/models/booking.cr#L151-L159

• Implement #as_json for 1, 2

defining to_json(json : JSON::Builder) would be the best way to go instead of to_h/as_json

It might be helpful to get some ideas from how active-model does it.
https://github.com/spider-gazelle/active-model/blob/master/src/active-model/model.cr#L62-L84

Originally posted by @caspiano in #80 (comment)

[caspiano]

@chillfox What method are you using for your serialisation subsets in PortalAPI?

[chillfox]

pretty much all of the controllers return json like this render :created, json: domain, type: ::PlaceOS::Portal::Models::Domain. If the returned object is a Clear model then the json method will be whatever Clear does, else there are some objects that use include JSON::Serializable for json.

[caspiano]

Ahh, I was looking at this - https://github.com/place-technology/portal-api/blob/develop/src/models/user.cr#L9

I think having a macro to generate types like this would be nice (and methods on the model to generate the serialisation intermediate)

[chillfox]

oh, yeah, there is that as well.
yep, some helper macros would be useful.