Introduce pg_dsm_registry_allocations view.

Introduce pg_dsm_registry_allocations view.

This commit adds a new system view that provides information about
entries in the dynamic shared memory (DSM) registry.  Specifically,
it returns the name, type, and size of each entry.  Note that since
we cannot discover the size of dynamic shared memory areas (DSAs)
and hash tables backed by DSAs (dshashes) without first attaching
to them, the size column is left as NULL for those.

Bumps catversion.

Author: Florents Tselai <florents.tselai@gmail.com>
Reviewed-by: Sungwoo Chang <swchangdev@gmail.com>
Discussion: https://postgr.es/m/4D445D3E-81C5-4135-95BB-D414204A0AB4%40gmail.com