Add support for "ip_prefix" bucket aggregation

`elasticsearch_dsl/aggs.py` is missing "ip_prefix" bucket aggregation which is introduced in [v8.1.0](https://www.elastic.co/guide/en/elasticsearch/reference/8.1/release-notes-8.1.0.html#bug-8.1.0). [IP prefix aggregation](https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-ipprefix-aggregation.html)