Elasticsearch Store Improvements
Labels: name: lgtm, name: area: vector store, name: auto:improvement

Todo:
- [x] Connection options (cloud, localhost url, es_connection) support
- [x] Logging support
- [x] Customisable field support
- [x] Distance Similarity support 
- [x] Metadata support
  - [x] Metadata Filter support 
- [x] Retrieval Strategies
  - [x] Approx
  - [x] Approx with Hybrid
  - [x] Exact
  - [x] Custom 
  - [x] ELSER (excluding hybrid as we are working on RRF support)
- [x] integration tests 
- [x] Documentation

👋 this is a contribution to improve Elasticsearch integration with Langchain. Its based loosely on the changes that are in master but with some notable changes:

## Package name & design improvements
The import name is now `ElasticsearchStore`, to aid discoverability of the VectorStore.

```py
## Before
from langchain.vectorstores.elastic_vector_search import ElasticVectorSearch, ElasticKnnSearch

## Now
from langchain.vectorstores.elasticsearch import ElasticsearchStore
```

## Retrieval Strategy support
Before we had a number of classes, depending on the strategy you wanted. `ElasticKnnSearch` for approx, `ElasticVectorSearch` for exact / brute force. 

With `ElasticsearchStore` we have retrieval strategies:

### Approx Example
Default strategy for the vast majority of developers who use Elasticsearch will be inferring the embeddings from outside of Elasticsearch. Uses KNN functionality of _search.

```py
        texts = ["foo", "bar", "baz"]
       docsearch = ElasticsearchStore.from_texts(
            texts,
            FakeEmbeddings(),
            es_url="http://localhost:9200",
            index_name="sample-index"
        )
        output = docsearch.similarity_search("foo", k=1)
```

### Approx, with hybrid
Developers who want to search, using both the embedding and the text bm25 match. Its simple to enable.

```py
 texts = ["foo", "bar", "baz"]
       docsearch = ElasticsearchStore.from_texts(
            texts,
            FakeEmbeddings(),
            es_url="http://localhost:9200",
            index_name="sample-index",
            strategy=ElasticsearchStore.ApproxRetrievalStrategy(hybrid=True)
        )
        output = docsearch.similarity_search("foo", k=1)
```

### Approx, with `query_model_id`
Developers who want to infer within Elasticsearch, using the model loaded in the ml node.

This relies on the developer to setup the pipeline and index if they wish to embed the text in Elasticsearch. Example of this in the test.

```py
 texts = ["foo", "bar", "baz"]
       docsearch = ElasticsearchStore.from_texts(
            texts,
            FakeEmbeddings(),
            es_url="http://localhost:9200",
            index_name="sample-index",
            strategy=ElasticsearchStore.ApproxRetrievalStrategy(
                query_model_id="sentence-transformers__all-minilm-l6-v2"
            ),
        )
        output = docsearch.similarity_search("foo", k=1)
```

### I want to provide my own custom Elasticsearch Query
You might want to have more control over the query, to perform multi-phase retrieval such as LTR, linearly boosting on document parameters like recently updated or geo-distance. You can do this with `custom_query_fn`

```py
        def my_custom_query(query_body: dict, query: str) -> dict:
            return {"query": {"match": {"text": {"query": "bar"}}}}

        texts = ["foo", "bar", "baz"]
        docsearch = ElasticsearchStore.from_texts(
            texts, FakeEmbeddings(), **elasticsearch_connection, index_name=index_name
        )
        docsearch.similarity_search("foo", k=1, custom_query=my_custom_query)

```

### Exact Example
Developers who have a small dataset in Elasticsearch, dont want the cost of indexing the dims vs tradeoff on cost at query time. Uses script_score.

```py
        texts = ["foo", "bar", "baz"]
       docsearch = ElasticsearchStore.from_texts(
            texts,
            FakeEmbeddings(),
            es_url="http://localhost:9200",
            index_name="sample-index",
            strategy=ElasticsearchStore.ExactRetrievalStrategy(),
        )
        output = docsearch.similarity_search("foo", k=1)
```

### ELSER Example
Elastic provides its own sparse vector model called ELSER. With these changes, its really easy to use. The vector store creates a pipeline and index thats setup for ELSER. All the developer needs to do is configure, ingest and query via langchain tooling.

```py
texts = ["foo", "bar", "baz"]
       docsearch = ElasticsearchStore.from_texts(
            texts,
            FakeEmbeddings(),
            es_url="http://localhost:9200",
            index_name="sample-index",
            strategy=ElasticsearchStore.SparseVectorStrategy(),
        )
        output = docsearch.similarity_search("foo", k=1)

```

## Architecture
In future, we can introduce new strategies and allow us to not break bwc as we evolve the index / query strategy. 

## Credit
On release, could you credit @elastic and @phoey1 please? Thank you!