How does one use the new Elasticsearch 7.7 Async Background search facility using the NEST Library 7.7
It seems this feature is available only in X-Pack.
Does one need to buy a separate licence if we are already have a cloud Elastic subscription?
The Async Search feature is part of the basic license if you install on-premises and part of the standard license if you use the elastic cloud, there is no need to buy or upgrade your license.
You can see the on-premises license differences here and the cloud subscription differences here
But, the Async Search API is not implemented in the NEST client yet, according to this github issue, which is still open.