[CBM] When populating staging directory we should not create files for directories

What is the problem?
In a customer issue, we found that cbbackupmgr does not work correctly when using an Azure container backed by DataLake Gen 2. One of the issues found was when populating the staging directory we would create a file for a directory. This is because IterateObjects will return directories and our population code assumes it doesn't - it always creates a file for every object given to it by IterateObjects.

What is the solution?
We should probably alter our staging directory code to skip creating files for directories (based on the IsDir flag).