Exposing index AM properties at SQL level

[Egor Rogov]

Hi,

I wonder if there is some hidden logic behind exposing (or not exposing) 
index AM properties at SQL level?

For example, should newly added amcanbuildparallel (b437571) be 
available via pg_indexam_has_property()? And also other boolean 
properties like ampredlocks or amusemaintenanceworkmem? I can't see a 
particular practical reason to expose them, but it looks inconsistently.

Thanks,
Egor