Add MultiTerms aggregation type

[chrispappas]

This library is missing a MultiTerms aggregation, which takes in multiple field names and returns buckets with the unique tuples matching.

https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-multi-terms-aggregation.html

A multi-bucket value source based aggregation where buckets are dynamically built - one per unique set of values.

Example:

GET /products/_search
{
  "aggs": {
    "genres_and_products": {
      "multi_terms": {
        "terms": [{
          "field": "genre" 
        }, {
          "field": "product"
        }]
      }
    }
  }
}

Possible method signature:

(new Builder($client))->addAggregation(
  MultiTermsAggregation::create('agg_name', ['field1', 'field2'])
);

[chrispappas]

PR is up for this feature:
#63