`ElasticsearchInferenceDenseRetriever` (ES inference for dense, optional)

[davidsbatista]

• Add a new retrieval method (e.g. _inference_dense_retrieval) that takes query text (and optional inference_id), calls the ES inference API to obtain the query vector, then runs kNN using the existing dense vector index.
• New retriever component: input query (text); calls the document store’s inference-based dense retrieval method.
• ES: Inference API, kNN search.