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.

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