[Feature Request] Add perplexity API support

### 🥰 需求描述

Perplexity API provides advanced capabilities for search and conversational AI. Adding support for this API will enable users to leverage its features directly within ChatGPT-Next-Web, enhancing the platform's functionality and expanding its use cases.

### 🧐 解决方案

Adding a configuration option in the settings to input API keys for Perplexity.
Implementing API calls to Perplexity endpoints within the backend.
Creating a UI toggle or button to switch between OpenAI and Perplexity responses.
Ensuring compatibility with existing features like chat history and response formatting.

### 📝 补充信息

Documentation for Perplexity API can be referenced for implementation details.
This feature can attract users who rely on Perplexity's unique capabilities, further diversifying the platform's user base.