[stable29] multi-instance object store tweaks

[leftybournes]

No description provided.

[icewind1991]

I'm not sure if a move command like this is the correct option. (Only setting the stored instance, not moving an actual object). Or if we want to extend https://github.com/nextcloud/multibucket_migrate instead, which does perform the object moving.

[icewind1991]

If we go with this version of the move command, I think it makes sense to validate that the target object store is valid for the user.

We could get the fileid for one of the user's files (if the user has at least one file) and check if an object with the matching urn exists it the target object store.

Without any checks the command would risk being a big footgun.

[icewind1991]

Outside of the move command the changes look good

[leftybournes]

I'm not sure if a move command like this is the correct option. (Only setting the stored instance, not moving an actual object). Or if we want to extend https://github.com/nextcloud/multibucket_migrate instead, which does perform the object moving.

I think extending multibucket_migrate is the way to go since that's already available instead of adding the move command.