Add support for "geohex_grid" bucket aggregation

`elasticsearch_dsl/aggs.py` is missing "geohex_grid" 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#feature-8.1.0). [Geohex grid aggregation](https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-geohexgrid-aggregation.html)