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/langchain_perplexity/__init__.py

@@ -0,0 +1,5 @@
+"""This package provides the Perplexity integration for LangChain."""
+
+from langchain_perplexity.chat_models import ChatPerplexity
+
+__all__ = ["ChatPerplexity"]