partners: Add Perplexity Chat Integration (#30618)

Perplexity's importance in the space has been growing, so we think it's
time to add an official integration!

Note: following the release of `langchain-perplexity` to `pypi`, we
should be able to add `perplexity` as an extra in
`libs/langchain/pyproject.toml`, but we're blocked by a circular import
for now.

---------

Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>
Co-authored-by: Chester Curme <chester.curme@gmail.com>

libs/partners/perplexity/README.md

@@ -0,0 +1,29 @@
+# langchain-perplexity
+
+This package contains the LangChain integration with Perplexity.
+
+## Installation
+
+```bash
+pip install -U langchain-perplexity
+```
+
+And you should [configure your perplexity credentials](https://docs.perplexity.ai/guides/getting-started)
+and then set the `PPLX_API_KEY` environment variable.
+
+## Usage
+
+This package contains the `ChatPerplexity` class, which is the recommended way to interface with Perplexity chat models.
+
+```python
+import getpass
+import os
+
+if not os.environ.get("PPLX_API_KEY"):
+  os.environ["PPLX_API_KEY"] = getpass.getpass("Enter API key for Perplexity: ")
+
+from langchain.chat_models import init_chat_model
+
+llm = init_chat_model("llama-3.1-sonar-small-128k-online", model_provider="perplexity")
+llm.invoke("Hello, world!")
+```