expose remoteId via share api

[tobiasKaminsky]

For nextcloud/android#14437

Summary

Android client relies on remote id to cache thumbnails.
Share api does not yet provide it, thus thumbnails are not properly generated or stored twice.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[tobiasKaminsky]

ToDo: check if etag is possible to add