Add tab completion support for WAIT FOR command

[Xuneng Zhou]

Hi hackers,

Thread [1] proposed the WAIT FOR command. This thread presents a
follow-up patch adding tab completion support, per Jian’s idea.

[1] Implement  tab completion for the WAIT FOR LSN command in psql.
https://www.postgresql.org/message-id/flat/CAPpHfdsjtZLVzxjGT8rJHCYbM0D5dwkO%2BBBjcirozJ6nYbOW8Q%40mail.gmail.com

v11-0001-Implement-WAIT-FOR-command.patch
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v1-0001-Add-tab-completion-support-for-WAIT-FOR-command.patch
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