Indicate which index is CLUSTER index

Currently when looking at table structure we cannot tell which index is the cluster index (or that clustering is as such being done). It would be comprehensive to show which index, if any, is the cluster index. I have to revert to psql with "\\d _table_" to find this out which is inconvenient.

Use case: more so now that RDS supports pg_repack, it becomes very valuable to setup table clustering, and among the DBs I manage, I like to quickly see that all tables are being clustered, and properly so.

Start from Postico's table structure view and compare its displayed information with what psql's "\\d _table_" reports. The work is done when the view clearly indicates which index, if any, is the table's cluster index, including when clustering is not configured.

Written by the indexing model from the issue text.