Add new multi_term aggregation (#2687)

Adds a new multi_term aggregation. The current implementation focuses 
on adding new type aggregates. Performance (latency) is suboptimal in this
iteration, mainly because of brute force encoding/decoding a list of values
into bucket keys. A performance improvement change will be made as a 
follow on.

Signed-off-by: Peng Huo <penghuo@gmail.com>

client/rest-high-level/src/main/java/org/opensearch/client/RestHighLevelClient.java

@@ -139,7 +139,9 @@
 import org.opensearch.search.aggregations.bucket.sampler.InternalSampler;
 import org.opensearch.search.aggregations.bucket.sampler.ParsedSampler;
 import org.opensearch.search.aggregations.bucket.terms.LongRareTerms;
+import org.opensearch.search.aggregations.bucket.terms.MultiTermsAggregationBuilder;
 import org.opensearch.search.aggregations.bucket.terms.ParsedLongRareTerms;
+import org.opensearch.search.aggregations.bucket.terms.ParsedMultiTerms;
 import org.opensearch.search.aggregations.bucket.terms.ParsedSignificantLongTerms;
 import org.opensearch.search.aggregations.bucket.terms.ParsedSignificantStringTerms;
 import org.opensearch.search.aggregations.bucket.terms.ParsedStringRareTerms;
@@ -2140,6 +2142,7 @@ static List<NamedXContentRegistry.Entry> getDefaultNamedXContents() {
         map.put(IpRangeAggregationBuilder.NAME, (p, c) -> ParsedBinaryRange.fromXContent(p, (String) c));
         map.put(TopHitsAggregationBuilder.NAME, (p, c) -> ParsedTopHits.fromXContent(p, (String) c));
         map.put(CompositeAggregationBuilder.NAME, (p, c) -> ParsedComposite.fromXContent(p, (String) c));
+        map.put(MultiTermsAggregationBuilder.NAME, (p, c) -> ParsedMultiTerms.fromXContent(p, (String) c));
         List<NamedXContentRegistry.Entry> entries = map.entrySet()
             .stream()
             .map(entry -> new NamedXContentRegistry.Entry(Aggregation.class, new ParseField(entry.getKey()), entry.getValue()))