Files: Support endpoint to retrieve total size of a directory.

[scblack321]

When implementing this, be sure to consider handling things like:

• follow or don't follow symlinks
• max depth
• don't fall victim to circular symlinks

Maybe use "du" command from ssh to get the results? Some or all of those issues are probably handled already

[scblack321]

This has also been requested by @NotChristianGarcia , so we might want to bump up the priority.
One option is to add an endpoint (or modify an existing endpoint) to run du -b for a path on a LINUX type Tapis system.