feat: Implement 'Bucket Sort Aggregation'

[dimatill]

Adds implementation of Bucket Sort Aggregation https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-pipeline-bucket-sort-aggregation.html

Usage example:

const reqBody = esb.requestBodySearch()
     .agg(
         esb.bucketSortAggregation('sort')
             .sort([
                  esb.sort('user', 'desc')
              ])
              .from(5)
              .size(10)
         )
     );

[sudo-suhas]

Only one minor change request, otherwise the PR is very thorough. Thank you!

[sudo-suhas]

I don't think it is necessary to repeat the URL for reference and example in every method's docs. Could you please remove them?

[XBeg9]

@sudo-suhas hi, any updates here? looking forward to getting this merged. Thanks.

[sudo-suhas]

Merged and released in elastic-builder@2.1.0 🎉

[XBeg9]

@sudo-suhas that's great! Thanks