Add support for environment proxies (HTTP, HTTPS, SOCKS) and improve Node.js proxy compatibility

[amirziveh]

This PR implements comprehensive proxy support for the fetch server, allowing it to respect standard environment variables and providing better compatibility across different runtimes (Bun and Node.js).

❌ The Problem
Previously, the server had limited proxy support:

Lack of Environment Support: The server ignored standard proxy environment variables (HTTP_PROXY, HTTPS_PROXY, ALL_PROXY), forcing users to manually pass the --proxy flag for every request.
Node.js Compatibility: The implementation relied on the proxy option in the fetch API. While this works in Bun, it is silently ignored by the native fetch implementation in Node.js, making proxies non-functional when running on Node.
Strict URL Requirements: The --proxy flag required a full URL including the protocol (e.g., http://...). Providing just an IP and port (e.g., 127.0.0.1:10808) resulted in "Invalid URL" errors.
Stream Incompatibility: Using node-fetch to solve the Node.js proxy issue introduced a crash in readResponseText because node-fetch returns Node.js Readable streams instead of the Web Streams expected by the existing code.
✅ The Solution
Environment Proxy Detection:
Implemented a new getProxyUrl helper that automatically detects and prioritizes proxy settings from HTTPS_PROXY, HTTP_PROXY, and ALL_PROXY (and their lowercase variants).
Multi-Protocol Support:
Integrated https-proxy-agent and socks-proxy-agent to provide robust support for HTTP, HTTPS, and SOCKS proxies.
Cross-Runtime Fetch Logic:
Bun: Continues to use the native proxy option for maximum performance.
Node.js: Now uses node-fetch combined with the appropriate proxy agent to ensure the proxy is correctly applied.
Smart URL Resolution:
Added logic to automatically prepend http:// to proxy strings that lack a protocol, allowing users to provide simple host:port strings.
Unified Stream Handling:
Updated readResponseText to detect the type of response body. It now supports both Web Streams (via getReader()) and Node.js Readable streams (via for await...of), ensuring stability across all environments.
🛠️ Changes
Modified src/Fetcher.ts to implement the new proxy resolution and fetch logic.
Added node-fetch, https-proxy-agent, and socks-proxy-agent as dependencies.
Verified that all existing tests pass and the CLI works correctly with both environment variables and the --proxy flag.