psql: Improve tab completion for COPY command.

psql: Improve tab completion for COPY command.

Previously, tab completion for COPY only suggested plain tables
and partitioned tables, even though materialized views are also
valid for COPY TO (since commit 534874fac0b), and foreign tables
are valid for COPY FROM.

This commit enhances tab completion for COPY to also include
materialized views and foreign tables.

Views with INSTEAD OF INSERT triggers are supported with
COPY FROM but rarely used, so plain views are intentionally
excluded from completion.

Author: jian he <jian.universality@gmail.com>
Co-authored-by: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Reviewed-by: David G. Johnston <david.g.johnston@gmail.com>
Discussion: http://postgr.es/m/CACJufxFxnSkikp+GormAGHcMTX1YH2HRXW1+3dJM9w7yY9hdsg@mail.gmail.com