Use postgres 12's support functions for function row/cost estimation

Currently we use the planner hook to adjust the cost for functions like `read_intermediate_result`. Postgres 12 [added](https://www.postgresql.org/docs/12/sql-createfunction.html) `SUPPORT support_function` syntax to `CREATE FUNCTION` with which we can provide cost/row estimation functions to postgres. See postgres'[ `src/include/nodes/supportnodes.h`](https://github.com/postgres/postgres/blob/REL_12_STABLE/src/include/nodes/supportnodes.h).

This syntax is not available in postgres 11, so we should probably wait until Citus support for postgres 11 is dropped.