ElasticsearchStore async methods implementation

[giacbrd]

The ElasticsearchStore class inherits the async methods implemented in VectorStore, that are actually not concurrent. The official Elasticsearch library now implements a helper for vector search, with both a sync and an async version, thanks to the developers of langchain-elastic!

The goal of this PR is to better integrate the helper in ElasticsearchStore, so that its async methods can be overridden with proper implementations.

I have considered different approaches, but there is critical difference in the design of the helper and the VectorStore class. The first has two completely independent classes for sync/async, the second exposes sync and async methods in the same abstraction. I am trying to follow the simplest approach possible, at the cost of:

• Creating two Elasticsearch clients in ElasticsearchStore, even when only one of sync or async approach is used
• Reducing the "availability" of the async methods, in order to make ElasticsearchStore robust on legacy code

Hope to receive feedback and improvement hints

[giacbrd]

The proposed solution makes use of two new parameters on the constructor of ElasticsearchStore. Their combination with other parameters it is not "linear", but the new parameters are unset by default. They activate the async implementations through the AsyncElasticsearch client and AsyncVectorStore. I think that async activation should be explicitly set when needed, because the use of AsyncElasticsearch can have some side effects on the process, that one must manage, e.g. closing the async connection.