feat: Add Perplexity citation support

[dbirks]

Pull Request Template

Summary

Taking a shot at adding citations for the Perplexity responses. Tried to take some design inspiration from the https://perplexity.ai site.

• Related discussion: Enhancement: Perplexity Completion response incl. citations #4692
• Associated librechat-agents PR: feat: Add Perplexity citation support agents#43

Please feel free to push to either of my branches to fix stuff up if that makes anything easier. Thanks!

Change Type

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• New feature (non-breaking change which adds functionality)

Testing

# Setup librechat-agents
gh pr checkout https://github.com/danny-avila/agents/pull/43
npm i
npm run build
npm link

# Setup LibreChat
gh pr checkout https://github.com/danny-avila/LibreChat/pull/XXX
npm i
npm link @librechat/agents
npm run frontend

# Run
docker start chat-mongodb
npm run backend

Checklist

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• My code adheres to this project's style guidelines
• I have performed a self-review of my own code
• I have commented in any complex areas of my code
• My changes do not introduce new warnings
• I have written tests demonstrating that my changes are effective or that my feature works
• Local unit tests pass with my changes