feat(fetch): add support for using proxy for requests

[shiquda]

Description

Server Details

• Server: fetch
• Changes to: requests

Motivation and Context

In certain network environments (e.g. in China), direct access to the target URL may be restricted. By allowing users to specify a proxy server, it can help the mcp-server-fetch service bypass these restrictions and function normally.

How Has This Been Tested?

I am in China and cannot directly access wikipedia.org. I use @modelcontextprotocol/inspector for testing, starting the mcp-server-fetch service with the --proxy-url parameter to specify the proxy URL and send requests containing the URL. The service can successfully retrieve the URL content. Without this parameter, it shows a request timeout.

Breaking Changes

no.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Protocol Documentation
• My changes follows MCP security best practices
• I have updated the server's README accordingly
• I have tested this with an LLM client
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have documented all environment variables and configuration options

Additional context

[longhaiqwe]

LGTM

[olaservo]

Created #1146 as a followup.