PoC - Avoid retrieving unnecessary fields on node-reduce phase

[carlosdelest]

Related: #134363

Avoids retrieving unnecessary fields in FieldExtractorExec on the node-reduce phase.

This PR checks that the fields retrieved are either:

• Present on the TopN operator on the node-reduce phase, or
• Needed on the top level Project for the node-reduce phase

This way, a query like:

FROM test METADATA _score
| WHERE knn(vector, [0, 1, 2])
| SORT _score DESC
| LIMIT 10
| KEEP id, _score

will not retrieve the vector field on the node-reduce phase and will avoid loading it completely from source.