We currently disallow a command's request parameter or reply field to have bson_serialization_type "any" because we cannot provide guarantees for its type compatibility for the stable API. However, there are still a lot of valid uses for bson_serialization_type "any", so we do need to support it and do basic compatibility checking for it.

We should have an allowList that determines which types can have the "any" type, so that any new usage is only intentionally added.