Kacy/postgres string agg implementation and group by chunks

[k-lombard]

• Implements functionality for queries on Document Tables of the type: GROUP BY '8 chunks'
• Implements functionality for STRING_AGG queries of the type:

SELECT STRING_AGG(paragraph || ' ' || name, '&') FROM MyPDFs GROUP BY '8 paragraphs';

• The "ORDER BY" optional third parameter for STRING_AGG is not yet supported.