Add `ParentFolderUid` to /api/search response

<!-- Please only use this template for submitting feature requests --> **Why is this needed**: Right now it's difficult to manage nested folders with the HTTP API. The /api/folders endpoint only returns top-level folders _or_ the direct children of a specific `parentUid`, but there's no way to tell whether a folder is nested without making many individual requests for each folder. The /api/search endpoint does support listing all folders at any level, but doesn't expose the folder hierarchy. Ideally you should be able to get the entire directory tree in a single API call. **What would you like to be added**: [Add a brief description of what the feature or enhancement should do.] Add a `folderParentUid` field to the /api/search result. This will give you one way to get the full directory structure in a single call. Though, this functionality should also be added to the folders API in some way as well. **Who is this feature for?** Users that manage dashboard folders with the API.