[YSQL] Support for PostgreSQL parallelism for Parallel Append plan node

[sushantrmishra]

Jira Link: DB-7132

Description

Support for PostgreSQL parallelism for Parallel Append.

https://www.postgresql.org/docs/11/parallel-plans.html#PARALLEL-APPEND
This feature should also be controllable using enable_parallel_append .

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[andrei-mart]

Append does not require any extra effort, it works as soon as generic parallel query works.
Handled by 2f91e3e, closing.