[BUG] Add support to parse sub-aggregations from nested aggregations[In SingleBucketAggregateBase.java the aggregations is not set in generator]

[Sangeetha14]

What is the bug?

As a part of Bug #233 I have found the issue - In SingleBucketAggregateBase.java the aggregations is not set in generator

How can one reproduce the bug?

I am trying to convert SearchResponse to Json. There it is trying serialize with SingleBucketAggregateBase.java. In this class aggregation is not set to generator.

What is the expected behavior?

In final SearchResponse Json, aggregation is should be available.

What is your host/environment?

I am using OpenSearch Version - 2.12, OpenSearch Client Version - 2.8.

Do you have any screenshots?

[dblock]

@Sangeetha14 want to give this a shot?