Tab completion for ATTACH PARTITION

Hi,

Currently, the psql's tab completion feature does not support properly 
for ATTACH PARTITION. When <TAB> key is typed after "ALTER TABLE 
<table_name> ATTACH PARTITION ", all possible table names should be 
displayed, however, foreign table names are not displayed. So I created 
a patch that addresses this issue by ensuring that psql displays not 
only normal table names but also foreign table names in this case.

Any kind of feedback is appreciated.

Best,
Tung Nguyen