Sync saved searches across clients

## Is your feature request related to a problem? Please describe.

ownCloud iOS app 12.0+ comes with a very fancy search feature:
- https://github.com/owncloud/ios-app/pull/1142

Users can combine multiple search parameters, then save this search to access it later again.

It would be super-awesome, if users could see the same search results as a shortcut in other clients, for example in the left sidebar in ownCloud web. (https://github.com/owncloud/web/issues/7691)

## Describe the solution you'd like

oCIS would need to provide a new API/endpoint, where such search queries could be saved and accessed from all clients. (per-user)

## Describe alternatives you've considered

File-based approach inside the WebDAV tree: iOS app could upload a hidden JSON file in a hidden `.search` folder.

## Additional context

See screenshots and example search queries in https://github.com/owncloud/ios-app/pull/1142