Add parentUid for creating folders

[dexoid]

Hello.
There is no way to create subfolder using this SDK. But according to official Grafana api documentation additional field for creating subfolders: parentUid.
https://grafana.com/docs/grafana/latest/developers/http_api/folder/#create-folder
Could you add it to grafana-api-sdk?
Thank you!

[ZPascal]

@dexoid Yes, I'll add it by the end of the week.

[ZPascal]

@dexoid This PR fix the case and adds the new feature. The PR is included inside release 0.6.1.