[REEF-2036] Add IFileSystem api to check if uri is a directory

[dwaijam]

[REEF-2036] Add IFileSystem api to check if uri is a directory

Currently, there is no way to check if a location specified by a uri is a directory or not. The only way to check it is by calling fs.GetChildren and checking for an exception. There should be a more graceful way of doing this check by adding an api. This change addresses this.

JIRA: REEF-2036

Pull request: This closes 1473