Feature/range aggregation fix 369

[szczepanczykd]

Description

Add the key property to RangeBucket, which fixes parsing range_date + range aggregation response.
Create Integration tests validating the change.

The "key" property is marked as @nullable to ensure backward compatibility.

Issues Resolved

#369

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[dblock]

Thanks and sorry for the delay for merging!

[VachaShah]

Tried backporting this to 2.x with #381 but the client fails to compile with the following error:

/home/runner/work/opensearch-java/opensearch-java/java-client/src/test/java/org/opensearch/client/opensearch/integTest/restclient/AggregationRequestIT.java:38: error: incompatible types: org.apache.hc.core5.http.HttpHost[] cannot be converted to org.apache.http.HttpHost[]
        return new RestClientTransport(buildClient(settings, hosts), new JacksonJsonpMapper());

Since the HttpHost added here is from Apache HttpClient 5 while that change is not backported to 2.x.

[VachaShah]

Tried backporting this to 2.x with #381 but the client fails to compile with the following error:

/home/runner/work/opensearch-java/opensearch-java/java-client/src/test/java/org/opensearch/client/opensearch/integTest/restclient/AggregationRequestIT.java:38: error: incompatible types: org.apache.hc.core5.http.HttpHost[] cannot be converted to org.apache.http.HttpHost[]
        return new RestClientTransport(buildClient(settings, hosts), new JacksonJsonpMapper());

Since the HttpHost added here is from Apache HttpClient 5 while that change is not backported to 2.x.

Fixed in commit 5bde404