OmhStorageClient: add extension to calculate folder size recursively

[TranceLove]

Summary

It's a commonly used function, better to provide as library extension instead of allowing end users (devs) to reinvent the wheel over and over.

Checklist:

• [ ] Documentation is up to date to reflect these changes
• [x] Created Unit tests

[TranceLove]

Closing PR - will raise new PR after pushing to this repo.