[YSQL] Support for PostgreSQL parallelism for Aggregation plan node

[sushantrmishra]

Jira Link: DB-7133

Description

Add support for PostgreSQL parallelism for Aggregation plan node.
https://www.postgresql.org/docs/11/parallel-plans.html#PARALLEL-AGGREGATION

Add support for pushdown of aggregates with PG parallelism.

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[andrei-mart]

The same as #19839, but other ticket has more detailed explanation.
Fixed by fd5136c