Constant value fields

[benjelloun]

It's often useful to associate key-value pairs at the level of a RecordSet. We propose doing that by adding a "value" property to Field that allows it to have a fixed value.

This is similar to the "data" property of RecordSet, that makes it possible to specify the values (records) contained in a RecordSet directly in the Croissant definition.

The "value" of a Field can be atomic, for Fields with a simple dataType, or it can be structured, in case the Field has SubFields. For the latter case, a JSON string can be used to represent the value.