Sparse vector storage in `ElasticsearchDocumentStore`

- Add optional init parameter `sparse_vector_field: str | None = None`.
- When set: add a `sparse_vector` mapping for that field and, in `write_documents()`, write each document’s `sparse_embedding` to that field.
- ES: [sparse_vector field type](https://www.elastic.co/guide/en/elasticsearch/reference/current/sparse-vector.html).
