Using setUserField might lead to an unbounded number of keys

Hi Julian, as I was debugging my changes for https://github.com/NodeBB/nodebb-plugin-sso-oauth2-multiple/pull/125 I've noticed that this line: ``` await db.setObjectField(`${payload.name}Id:uid`, payload.oAuthid, uid); ``` ( https://github.com/NodeBB/nodebb-plugin-sso-oauth2-multiple/blob/main/library.js#L251 ) will add a key to a single mongo document for each user. I'm afraid that when there will be hundreds of thousands users, things might be slow or even break. Is my concern valid, or is it handled somewhere? Thanks, Tomas