Allow querying on fields that are mapped to sparse column

[CraigLRussell]

Currently, query only allows fields that are mapped to their own columns. Fields that are mapped to sparse columns cannot be used in query expressions. This restriction is ok for now, since there is no way to push the query to the back end.

But in future, it will be possible to create query expressions that include fields from the column with the sparse fields, by using the JSON functions in the server. It will even be possible to index some of the fields from the sparse columns.