Add support for composite

[Sander2k]

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Hi, I'd be very interested in having the support for composite as in https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-composite-aggregation.html

I get the error Unknown BaseAggregationBuilder [composite] and it seems that I am not the only one, see also: https://github.com/ropensci/elastic/issues/227

If it should work in a different way or another package, please let me know. Rest works fine, thank you for the package! Kind regards Sander2k