WebDAV property to help minimize requests for previews

## Is your feature request related to a problem? Please describe.
Currently, there is no way for clients to tell if a preview (thumbnail) is available - or even *likely* to be available - for a file. Therefore clients need to request previews for every file. Depending on the type of stored files this can mean a lot of unnecessary requests and therefore client and server-side overhead.

## Describe the solution you'd like
An additional `PROPFIND` item property `oc:request-preview`, which signals to the client that it should or shouldn't request a preview for an item.

## Describe alternatives you've considered
Alternative 1:
- idea: clients observe for which file types they didn't get a preview - and stop requesting previews for the files of the same type.
- caveat: works as long as previews are consistently available or unavailable for a file type, but stops working the moment no preview can be generated for a file (f.ex. due to errors in the file or a wrong suffix) for which previews would typically be available.

Alternative 2:
- idea: provide clients with a list of file types for which previews are available, via capabilities or as drive properties (via Graph API)
- caveat: availability of previews may vary depending on underlying storage