lsfd: use ARRAY_STRING for ENDPOINTS column in JSON output mode

lsfd: use ARRAY_STRING for ENDPOINTS column in JSON output mode

This is the backport-for-v2.39 version of #2328.

With this change:
  {
     "lsfd": [
        {
           "assoc": "3",
           "endpoints": [
               "9942,test_mkfds,4-w", "9942,test_mkfds,5r-", "9942,test_mkfds,6-w"
           ]
        }
     ]
  }

Without this change:
 {
     "lsfd": [
        {
           "assoc": "3",
           "endpoints": [
               "9942,test_mkfds,4-w\n9942,test_mkfds,5r-\n9942,test_mkfds,6-w"
           ]
        }
     ]
  }

Signed-off-by: Masatake YAMATO <yamato@redhat.com>