Feature: deepseek provider

[HairlessVillager]

增加了对 DeepSeek 的支持，需要在 FellouAI/eko#41 合并之后进行测试和合并。

验证方法：

1. 编译 eko 和 eko-browser-extension-template 项目；
2. 在浏览器重新加载插件；
3. 打开插件选项，可以在LLM一栏中选中“DeepSeek”，选中后 Base URL 一栏自动填写为“https://api.deepseek.com”，Model 一栏自动填写为“DeepSeek V3 (default)”，可选的另一个 Model 为“DeepSeek R1”
4. 插件的其他行为应与在使用其他模型时并无二样。

需要注意，DeepSeek 官方的 API 近期不稳定，且加入了风控机制，需要关闭 VPN 使用。

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Added support for DeepSeek, which requires testing and merging after the pull request at FellouAI/eko#41 is merged.

Verification Method:

1. Compile the eko and eko-browser-extension-template repo;
2. Reload the extension in the browser;
3. Open the extension options and select "DeepSeek" in the LLM section. Upon selection, the Base URL field will automatically fill with "https://api.deepseek.com", and the Model field will automatically fill with "DeepSeek V3 (default)", with "DeepSeek R1" being another optional model.
4. The other behaviors of the extension should be the same as when using other models.

Please note that the official API of DeepSeek has been unstable recently and has incorporated risk control mechanisms. Therefore, it is necessary to disable VPN when using it.