IMPROVEMENT Increase flexibility of ElasticVectorSearch (#6863)

Hey @rlancemartin, @eyurtsev ,

I did some minimal changes to the `ElasticVectorSearch` client so that
it plays better with existing ES indices.

Main changes are as follows:

1. You can pass the dense vector field name into `_default_script_query`
2. You can pass a custom script query implementation and the respective
parameters to `similarity_search_with_score`
3. You can pass functions for building page content and metadata for the
resulting `Document`

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docs/docs/integrations/vectorstores/elasticsearch.ipynb

@@ -776,6 +776,40 @@
     "print(results[0])"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "# Customize the Document Builder\n",
+    "\n",
+    "With ```doc_builder``` parameter at search, you are able to adjust how a Document is being built using data retrieved from Elasticsearch. This is especially useful if you have indices which were not created using Langchain."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "from typing import Dict\n",
+    "from langchain.docstore.document import Document\n",
+    "\n",
+    "def custom_document_builder(hit: Dict) -> Document:\n",
+    "    src = hit.get(\"_source\", {})\n",
+    "    return Document(\n",
+    "        page_content=src.get(\"content\", \"Missing content!\"),\n",
+    "        metadata={\"page_number\": src.get(\"page_number\", -1), \"original_filename\": src.get(\"original_filename\", \"Missing filename!\")},\n",
+    "    )\n",
+    "\n",
+    "results = db.similarity_search(\n",
+    "    \"What did the president say about Ketanji Brown Jackson\",\n",
+    "    k=4,\n",
+    "    doc_builder=custom_document_builder,\n",
+    ")\n",
+    "print(\"Results:\")\n",
+    "print(results[0])"
+   ]
+  },
   {
    "cell_type": "markdown",
    "id": "3242fd42",
@@ -929,7 +963,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.10.3"
+   "version": "3.9.7"
   }
  },
  "nbformat": 4,