Add an endpoint to retrieve a folder index key based on the path

[reganwolfrom]

GET /folders?path={folderPath} instead of GET /folders?indexKey

Ideally, we would get both the folders and documents within, but would also include the current folder's indexKey and path, to enable a translation between folderPath and indexKey.

Also, a property to only show folders would be good, to remove the need for paging through documents when only folders are desired. Recommended property name is foldersOnly with true, defaulting to false when not provided.