WebDAV: expose MetaVox metadata using a SabreDAV plugin

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Feature request

Which Nextcloud Version are you currently using:

33.0.5

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

We are integrating our Nextcloud instance with an external document synchronization and indexing tool.

Currently, file content and properties are available through WebDAV, while MetaVox metadata are exposed through its OCS API only.

This implies that, when syncing a file, we have to issue a dedicated OCS API request to get MetaVox metadata.

Describe the solution you'd like

Expose file-level MetaVox metadata fields as WebDAV properties, for example {urn:metavox}gf_source

Expected behaviors:

• properties available via PROPFIND
• values resolved dynamically from MetaVox storage
• efficient implementation for collections/folders by using preloading to avoid N+1 queries (as suggested in the documentation)

This can be achieved by registering a SabreDAV plugin.

Describe alternatives you've considered

• Using the MetaVox OCS API in addition to WebDAV to fetch required data. This works but requires additional queries.
• Synchronizing MetaVox values into Nextcloud Files Metadata using Files Metadata API. While this seems possible, it also seems to be way heavier to implement.
• Implementing a custom app on our side that registers this SabreDAV plugin. But native support in MetaVox would be easier to maintain and more reusable.

Additional context

Nextcloud already extends the DAV server with custom properties through SabreDAV plugins (systemtag for instance).

Our use case is read-only synchronization for a structured document base used by external indexing.

Start with Nextcloud's DAV extension documentation and the existing systemtag SabreDAV plugin mentioned in the issue. Trace how MetaVox values are read from storage and how PROPFIND properties are registered, then define the read-only property behavior and collection preloading needed to avoid N+1 queries. Done means MetaVox fields are available through WebDAV for files and folders without extra per-item lookups.