[rql] Store selected variables for RQL select queries in ResultSet (#17218476)
By storing the name of the selected variables for RQL select queries in the ResultSet (within the "variables" attribute), the information can be passed down to specific protocols, e.g. rqlio that may wish to pass is down further to clients. In turn, clients can then choose to present the results of RQL select queries as symbolic bindings using the names used in the query's projection, instead of ordinal arrays.
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