Add non-global (i. e. private and group-related) tags

Is your feature request related to a problem? Please describe.
Currently tags are global and public (as in visible and accessible to all users). There is an obvious need for private tags as additional tool for file grouping/sorting per user or group. Members of a group might tag files as "urgent" or "not too bright an idea" without this to be visible to the rest of the user base. There might be Nextcloud instances open to employees and third parties (e. g. clients or external services). And I don't want to see my accountant that I've been tagging files with "cool porn" if I can avoid it but I also don't want to set up a second server just to share my files with him.

Global tags are more like a property for the workflow state engine or other global applications but not for file management.

Describe the solution you'd like
Add a new class of tags (that might even correspond to the already possible tagging using WebDAV) of files that are unique to groups (consider the groups their namespaces). Match them to WebDAV properties if possible. Add support for clients (many Linux file systems, Windows file systems and MacOS file systems support the notion of additional meta data anyway).

Describe alternatives you've considered
I tried prefixing tag names by "group-" but that does not make them invisible to the rest of the users. So tagging my photos from my last holidays with "Spain 2022" did not stay hidden from my colleagues even though they could not access the files I marked like that.
Comments are not alternative either.

No files, tests, or entry points are named in the issue. Begin by locating the existing global-tag and WebDAV-property implementations, then determine how user- and group-scoped visibility is represented. Done means private and group-related tags are hidden from unauthorized users while supporting the requested file-management and client metadata behavior.