Support sparse fieldsets (?fields=id,title)

[ryanmitchell]

In theory this is supported by Fractal (ref: https://github.com/thephpleague/fractal/pull/496/commits/2c65714138820c68e827546f3c0980755409692a)

I haven't tested this throughly so we shouldn't document it at this point, but we can treat it available in 'shadow' model for now?