feat(sync) Add a `belongsTo` property in the schema

[almet]

This new property allows to know what's the relation with the schema property and the different object types (map, datalayer, feature).

This serves two goals:

1. Check that the incoming messages make sense (it's possible to change this property in this object)
2. Document where the property is used in practice. Because there is a lot of properties, it's hard to keep track of where they are used. This makes it explicit.

This is to be merged after #1754