Add a postgis_geos_noop function

[strk]

For debugging/profiling reasons, it would be useful to have a function to just convert postgis geometries to geos and back. The lwgeom_geos_noop function already exists at the liblwgeom level, so would be a mattef or exposing it at the sql level.

[robe]

Milestone renamed

[strk]

In 16791:

Add postgis_geos_noop function

Includes regress test.
Closes #2902