Macro-based COSTS for version specific costing

[pramsey]

Because we cannot fully cost functions until Pg12/PostGIS 3, we need to macro-ize all costing, and then set higher costs on the new version case.

[pramsey]

In 17307:

Add costs to functions in the form of macros:
_COST_LOW, _COST_MEDIUM, _COST_HIGH
Macros are set in sqldefines.h.in and are switched
on PgSQL version. Prior to PG12, costs should be
small or nonexistent, to avoid SQL inlining issues.
After Pg12, costs can be higher to hopefully
force more parallelism in plans.
Closes #4344