Add a "Passive bool" to "description.Server"

We have a use case where we'd like to monitor a replset and track the number of up-to-date voting secondary data nodes. description.Server already exposes Kind and LastUpdateTime, but I don't believe we can distinguish between a voting and non-voting passive node (i.e. votes=0, priority=0, but non-hidden). Afaict the only difference in "isMaster" output is a "passive": true field. It'd be nice if this was parsed and saved to "description.Server", similar to ReadOnly.