Jonakemon

Add support for adjacency matrix

2

Closes #1553

When one currently tries to use the [adjacency matrix](https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-adjacency-matrix-aggregation.html) aggregation, one gets this error: `elasticsearch_dsl.exceptions.UnknownDslObject: DSL class 'adjacency_matrix' does not exist in agg.` This is not a bug, but a...