Bedrock embeddings async methods (#9024)

## Description
This PR adds the `aembed_query` and `aembed_documents` async methods for
improving the embeddings generation for large documents. The
implementation uses asyncio tasks and gather to achieve concurrency as
there is no bedrock async API in boto3.

### Maintainers
@agola11 
@aarora79  

### Open questions
To avoid throttling from the Bedrock API, should there be an option to
limit the concurrency of the calls?

docs/extras/integrations/llms/bedrock.ipynb

@@ -31,12 +31,11 @@
    },
    "outputs": [],
    "source": [
-    "from langchain.llms.bedrock import Bedrock\n",
+    "from langchain.llms import Bedrock\n",
     "\n",
     "llm = Bedrock(\n",
     "    credentials_profile_name=\"bedrock-admin\",\n",
-    "    model_id=\"amazon.titan-tg1-large\",\n",
-    "    endpoint_url=\"custom_endpoint_url\",\n",
+    "    model_id=\"amazon.titan-tg1-large\"\n",
     ")"
    ]
   },

docs/extras/integrations/text_embedding/bedrock.ipynb

@@ -20,7 +20,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": null,
+   "execution_count": 5,
    "id": "282239c8-e03a-4abc-86c1-ca6120231a20",
    "metadata": {},
    "outputs": [],
@@ -28,7 +28,7 @@
     "from langchain.embeddings import BedrockEmbeddings\n",
     "\n",
     "embeddings = BedrockEmbeddings(\n",
-    "    credentials_profile_name=\"bedrock-admin\", endpoint_url=\"custom_endpoint_url\"\n",
+    "    credentials_profile_name=\"bedrock-admin\", region_name=\"us-east-1\"\n",
     ")"
    ]
   },
@@ -49,7 +49,29 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "embeddings.embed_documents([\"This is a content of the document\"])"
+    "embeddings.embed_documents([\"This is a content of the document\", \"This is another document\"])"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "9f6b364d",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "# async embed query\n",
+    "await embeddings.aembed_query(\"This is a content of the document\")"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "c9240a5a",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "# async embed documents\n",
+    "await embeddings.aembed_documents([\"This is a content of the document\", \"This is another document\"])"
    ]
   }
  ],
@@ -69,7 +91,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.10.11"
+   "version": "3.9.13"
   }
  },
  "nbformat": 4,