pgsql: Add support for tab completion after ALTER EXTENSION ADD|DROP in

Add support for tab completion after ALTER EXTENSION ADD|DROP in psql

This appends the set of object types supported by these commands, and
the objects defined in the cluster are completed after that.  Note that
these may not be in the extension being working on when using DROP, to
keep the code simple, but this is much more useful than the previous
behavior of not knowing the objects that can be touched.

Author: Vignesh C
Discussion: https://postgr.es/m/CALDaNm3LVM2QcUWqgOonKZH80TveT-tUthbw4ZhuE_6pD3yi-A@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/2ff5ca86e816846743b31279a9f6b819d4cf4b11

Modified Files
--------------
src/bin/psql/tab-complete.c | 21 +++++++++++++++++++++
1 file changed, 21 insertions(+)