Function for listing pg_wal/summaries directory

Hi,

While WAL summaries feature and some support functions have been added 
in version 17, merely listing the contents of the pg_wal/summaries 
directory is missing. As discussed in the pg_ls_archive_status() 
discussion, it would be convenient to add pg_ls_summariesdir() that 
lists the contents of the pg_wal/summaries directory with the pg_monitor 
role.

This patch is based on a very recent master and does not include the 
catversion bump.

Best,
Yushi