copilot: Support HTTP/HTTPS proxy for Copilot language server

[eli-kaplan]

Closes #6701 (one of the top ranking issues as of writing)

Adds the ability to specify an HTTP/HTTPS proxy to route Copilot code completion API requests through. This should fix copilot functionality in restricted network environments (where such a proxy is required) but also opens up the ability to point copilot code completion requests at your own local LLM, using e.g.:

• https://github.com/jjleng/copilot-proxy
• https://github.com/bernardo-bruning/ollama-copilot/tree/master

External MITM-proxy tools permitting, this can serve as a stop-gap to allow local LLM code completion in Zed until a proper OpenAI-compatible local code completions provider is implemented. With this in mind, in this PR I've added separate settings.json variables to configure a proxy server specific to the code completions provider instead of using the global proxy setting, to allow for cases like this where we only want to proxy e.g. the Copilot requests, but not all outgoing traffic from the application.

Currently, two new settings are added:

• inline_completions.copilot.proxy: Proxy server URL (HTTP and HTTPS schemes supported)
• inline_completions.copilot.proxy_no_verify: Whether to disable certificate verification through the proxy

Example:

"features": {
  "inline_completion_provider": "copilot"
},
"show_completions_on_input": true,
// New:
"inline_completions": {
  "copilot": {
    "proxy": "http://example.com:15432",
    "proxy_no_verify": true
  }
}

Release Notes:

• Added the ability to specify an HTTP/HTTPS proxy for Copilot.

[eli-kaplan]

Thanks for helping clean this up! Let me know if you've got further changes planned or if you'd like me to go in and fix up the merge conflicts

[maxdeviant]

Thank you!

[sa1]

I would suggest falling back upon the global proxy setting if inline_completions.copilot.proxy is absent.
While some people would definitely want to run ollama, most other people in corporate environments would be perplexed to understand why their global proxy setting is not working, leading to a very bad user experience.
Most of this stuff is not documented anywhere except in commit messages.

[isav48]

the correct syntax:
"edit_predictions": { "copilot": { "proxy": "http://localhost:8081", "proxy_no_verify": true } },

[notpeter]

@eli-kaplan: "...instead of using the global proxy setting"

@sa1: "I would suggest falling back upon the global proxy setting if inline_completions.copilot.proxy is absent."

I wholly agree this should be the default behavior. If proxy is defined settings or the appropriate environment variables are set (HTTP_PROXY, SOCKS_PROXY, etc) that should be used for copilot by default.