Implement mongoc_bulkwrite_new() and mongoc_bulkwrite_set_client() in order to allow constructing a mongoc_bulkwrite_t without assigning a client. The internal client and operation_id are not used until mongoc_bulkwrite_execute(), so there is no technical impediment to such an API.

mongoc_bulkwrite_execute() will also need to be changed to assert or otherwise require that client and operation_id are set at the time it is called.

This API would follow prior art from mongoc_bulk_operation_t (namely mongoc_bulk_operation_new() and mongoc_bulk_operation_set_client()), which exists for the benefit of wrapping drivers such as PHP.

Note: I failed to catch this when reviewing the original implementation in mongodb/mongo-c-driver#1590.