We have seen in logv2 cleanup that there are places that are not in the mongo namespace lexical scope, but need our UDLs like `::mongo::operator"" _attr`.

We need to add a using directive to get the `_attr` UDL. We would NOT want to bring in all of ::mongo though. We should do what std does, and make a nested inline namespace: ::mongo::literals to hold our UDLs so you can get them without bringing in other ::mongo names. We could go further and make namespaces further nested inside literals like std::literals::string_literals and std::literals::chrono_literals, but I don't think we need to go that far. mongo only has a few UDLs.

This will help us introduce logv2 into arbitrary mongo code.