[FEATURE] Support for Neural search

[savanbthakkar]

Is your feature request related to a problem?

Not a problem with existing OpenSearchClient

What solution would you like?

With OpenSearch 2.9, we have the ability to search using neural search plugin.
(https://opensearch.org/docs/latest/search-plugins/neural-search/)
However, SearchDescriptor in OpenSearch.Client does not have the ability to use neural search.
It would be great to be able to use neural search in OpenSearch.Client since many OpenSearch users will be using Neural plugin since it's generally available in OS 2.9.

What alternatives have you considered?

A workaround for now may be to use low level opensearch client.

Do you have any additional context?

[Xtansia]

This will have similarities to how k-NN search was added, so this PR #215 will likely serve as a good reference point if someone decides to pick this up

[Xtansia]

I'm adding a working sample of how you could achieve this in the client currently using the "raw" methods in #614