Copilot Extentions agent Support

[biosugar0]

GitHub recently introduced Copilot Extensions:
https://docs.github.com/en/copilot/building-copilot-extensions/about-building-copilot-extensions

For instance, if a user enables the Perplexity AI extension, they can send requests to the Perplexity API directly from Copilot.

It would be extremely useful if CopilotChat.nvim could support calling these agents as well. In VS Code, this can be done by using the @ prefix, such as @perplexityai.

In my testing with Neovim, I found that Perplexity can be called with the Copilot token by implementing this approach:
#465

In the case of Perplexity, responses include content.copilot_references, so displaying referenced URLs and titles could enhance the user experience. I believe this addition would integrate seamlessly with the current implementation and add significant convenience!

In addition, sending a GET request to https://api.githubcopilot.com/agents retrieves a list of agents available to the user.

[deathbeam]

Thanks for your initial PR @biosugar0, was super helpful. Created PR #490 that implements this feature, together with @agent (and changed @buffer to #buffer etc).

Also added reference listing too: