feat(RAG): add BM25 Retriever.

[Aries-ckt]

Close #1577

Description

Add Elasticsearch BM25Retriever.
https://www.elastic.co/guide/en/elasticsearch/reference/8.13/index-modules-similarity.html

How Has This Been Tested?

1. install Elasticsearch refer https://www.elastic.co/guide/en/elasticsearch/reference/8.13/setup.html
2. update Elasticsearch config

def _create_es_config():
    """create Elasticsearch config."""
    return ElasticsearchVectorConfig(
        name="bm25_es_dbgpt",
        uri="localhost",
        port="9200",
        user="elastic",
        password="dbgpt",
    )

3. run example

 python examples/rag/bm25_retriever_example.py

Snapshots:

Include snapshots for easier review.

Checklist:

• My code follows the style guidelines of this project
• I have already rebased the commits and make the commit message conform to the project standard.
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• Any dependent changes have been merged and published in downstream modules

[csunny]

r+

[fangyinc]

LGTM.