Improve `MongoRepository.saveAll` to use batch updates

MongoDB supports efficient batch operations through bulkWrite, and SimpleMongoRepository.saveAll(...) already performs a batch insert when all entities are new .

However, if entities contains any non-new entity, it calls SimpleMongoRepository.save(...) for each entity, which seems neither efficient nor consistent.

A possible improvement would be something like the following:

List<S> newEntities = new ArrayList<>();
List<S> existingEntities = new ArrayList<>();

// ...

if (!newEntities.isEmpty()) {
   mongoOperations.insert(newEntities, entityInformation.getCollectionName());
}

if (!existingEntities.isEmpty()) {

   BulkOperations bulkOps = mongoOperations.bulkOps(BulkMode.ORDERED, entityInformation.getJavaType(),
            entityInformation.getCollectionName());

   for (S entity : existingEntities) {
      Query query = new Query(where(entityInformation.getIdAttribute()).is(entityInformation.getId(entity)));
      bulkOps.replaceOne(query, entity, FindAndReplaceOptions.options().upsert());
   }

   bulkOps.execute();
}

Start in SimpleMongoRepository.saveAll(...) and compare its all-new batch insert path with the per-entity save path for mixed collections. Read the MongoOperations bulk-operation entry points shown in the issue, then verify that new entities remain batch inserted and existing entities are replaced through one bulk operation with the intended upsert behavior.