community: add new parameter default_headers (#28700)

Thank you for contributing to LangChain!

- [x] **PR title**: "package: description"
- "community: 1. add new parameter `default_headers` for oci model
deployments and oci chat model deployments. 2. updated k parameter in
OCIModelDeploymentLLM class."


- [x] **PR message**:
- **Description:** 1. add new parameters `default_headers` for oci model
deployments and oci chat model deployments. 2. updated k parameter in
OCIModelDeploymentLLM class.


- [x] **Add tests and docs**:
  1. unit tests
  2. notebook

---------

Co-authored-by: Erick Friis <erick@langchain.dev>

docs/docs/integrations/chat/oci_data_science.ipynb

@@ -137,7 +137,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 1,
+   "execution_count": null,
    "metadata": {},
    "outputs": [],
    "source": [
@@ -156,6 +156,10 @@
     "        \"temperature\": 0.2,\n",
     "        \"max_tokens\": 512,\n",
     "    },  # other model params...\n",
+    "    default_headers={\n",
+    "        \"route\": \"/v1/chat/completions\",\n",
+    "        # other request headers ...\n",
+    "    },\n",
     ")"
    ]
   },