Allow nested aggregation to accept an Aggregation instance.

[ajjahn]

For example:

nested = Aggregation.new do
  sum field: 'foo'
end

outer = Aggregation.new do
  aggregation(:nested_agg, nested)
end

This matches the behavior of the top level query aggregation method.

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[cla-checker-service]

💚 CLA has been signed

[ajjahn]

Contributor Agreement has been signed. Any feedback or changes needed to make this mergeable?

[picandocodigo]

Thanks for this contribution @ajjahn! I'll review it this week!

[picandocodigo]

Transferred to the new repository.