Implement JSON_OBJECTAGG

JSON_OBJECTAGG is a function that makes JSON object out of a sequence of column or literal values.
Its algorithm is simplier comparing to JSON_ARRAYAGG but it can produce an enormous string in the end.
JSON_OBJECTAGG is an aggregation function by its definition thus it must be impemented as an aggregation function, e.g. group_concat w/o ORDER BY, LIMIT etc.