Mongo custom expressions (#17117)

Mongo custom expressions now are turned on for mongo 5.0 and after only. One idiosyncrasy is that we don't really support the BSON ID format at all in our typesystem despite having it in there, but both of my attempts to get them to work with the native mongo pipeline commands bounced because they really aren't strings, they're BSON ID's. We do rely on the pipeline commands heavily which is the reason for the version requirement.