psql: Fix tab completion for FOREIGN DATA WRAPPER and SUBSCRIPTION.

psql: Fix tab completion for FOREIGN DATA WRAPPER and SUBSCRIPTION.

Commit 8185bb5347 extended the CREATE/ALTER SUBSCRIPTION and
CREATE/ALTER FOREIGN DATA WRAPPER commands, but missed the
corresponding tab-completion logic. This commit fixes that oversight
by adding completion support for:

- The CONNECTION keyword in CREATE/ALTER FOREIGN DATA WRAPPER.
- The list of foreign servers in CREATE/ALTER SUBSCRIPTION.

Author: Yamaguchi Atsuo <acrobatcoder@gmail.com>
Discussion: https://postgr.es/m/CAKSyusJWdWcUKVd3qJXcEaQxJewGymQWV_r3-mc=Knrqo0AZ_g@mail.gmail.com