Replace use of index options in API with BSON document

Currently, we use mongocxx::options::index to pass options to our index-related API methods. This has the downside of requiring changes whenever the server begins supporting new options. To mitigate this, we should change the index-related API methods to take a BSON document rather than an options object. As part of the work, we should implement a cast operator to bsoncxx::document::value for mongocxx::options::index to enable existing code to continue working.