Use PgSQL12 "support function" API for indexed functions

[pramsey]

The PgSQL "support function" API allows functions to be re-written during the planning phase to include appropriate index operators.

​https://github.com/postgres/postgres/blob/93473c6ac805994a74e74ed13828c6c9433c8faf/src/include/nodes/supportnodes.h

Using this API will allow many of the SQL wrapper functions to be retired and for spatial functions to be more appropriately costed.

[pramsey]

In 17298:

Use "support function" API in PgSQL 12+ to provide index support for ST_Intersects and company, in place of old SQL inlining method.
Closes #4341

[pramsey]

In 17299:

Oops, add file to svn
References #4341