[vibe-d/mongodb] No read preference tag support

[gedaiu]

Can't target specific secondaries by tags like {dc: "east"} or {role: "analytics"}. The tags field is already parsed in ServerDescription but nothing uses it.

Need to parse readPreferenceTags from the connection URI and filter candidates in selectServer() by matching tag sets before picking one.

Depends on the per-operation read preference. #2848