Add support for Perplexity's PPLX models: pplx-7b-online and pplx-70b-online

[JasonWeill]

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Problem

Perplexity's PPLX models provide high-quality, frequently updated responses to prompts.

Proposed Solution

Using LangChain (see below), allow users to send magic command and chat UI prompts to PPLX models.

Additional context

https://github.com/langchain-ai/langchain/issues/14079 covers adding PPLX model support to LangChain; we should block this issue until issue 14079 is resolved and included in a LangChain release.

[jaiamin]

I am currently working on a PR for issue:14709 and expect it to be done within the next week or two.

[omerholz]

That's great @jamino30 I'm also interested in perplexity support. I'd love to help push your PR over the finish line.