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/tests/unit_tests/test_secrets.py

@@ -0,0 +1,8 @@
+from langchain_perplexity import ChatPerplexity
+
+
+def test_chat_perplexity_secrets() -> None:
+    model = ChatPerplexity(
+        model="llama-3.1-sonar-small-128k-online", pplx_api_key="foo"
+    )
+    assert "foo" not in str(model)