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/core/langchain_core/load/load.py

@@ -28,6 +28,7 @@ DEFAULT_NAMESPACES = [
     "langchain_fireworks",
     "langchain_xai",
     "langchain_sambanova",
+    "langchain_perplexity",
 ]
 # Namespaces for which only deserializing via the SERIALIZABLE_MAPPING is allowed.
 # Load by path is not allowed.