psql: Add leakproof field to \dAo+ meta-command results

Hi,

I would like to propose to add a new field to psql's \dAo+ meta-command
to show whether the underlying function of an operator is leak-proof.

This idea is inspired from [1] that claims some indexes uses non-LEAKPROOF
functions under the associated operators, as a result, it can not be selected
for queries with security_barrier views or row-level security policies.
The original proposal was to add a query over system catalogs for looking up
non-leakproof operators to the documentation, but I thought it is useful
to improve \dAo results rather than putting such query to the doc.

The attached patch adds the field to \dAo+ and also a description that
explains the relation between indexes and security quals with referencing
\dAo+ meta-command.

[1] https://www.postgresql.org/message-id/raw/5af3bf0c-5e0c-4128-81dc-084c5258b1af%40code406.com

Regards,
Yugo Nagata

--
Yugo NAGATA <nagata(at)sraoss(dot)co(dot)jp>