psql: show default extension version in \dx output

psql: show default extension version in \dx output

Reviewed-By: Julien Rouhaud <rjuju123@gmail.com>
Reviewed-By: Michael Banck <mbanck@gmx.net>
Reviewed-By: Yugo Nagata <nagata@sraoss.co.jp>
Reviewed-By: Nathan Bossart <nathandbossart@gmail.com>
Reviewed-By: Jelte Fennema-Nio <postgres@jeltef.nl>
Discussion: https://postgr.es/m/CABUevEyTMyXC6OvCWkj+rPnHrfi8_Rw_+DD_jzgFFNPqgf+Oig@mail.gmail.com