Add support for host allowlisting to Fetch server

Is your feature request related to a problem? Please describe. The fetch MCP server currently allows connections to local/internal IP addresses without any restrictions, which poses a security risk. While the README has been updated with a caution notice, this reference implementation could be misused if deployed without proper safeguards.

Describe the solution you'd like Implement a host allowlisting mechanism for the fetch server that would:

• Allow administrators to specify which hosts/domains the server can connect to
• By default, block connections to local/internal IP ranges (127.0.0.0/8, 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, etc.)
• Provide configuration options to explicitly allow internal IPs if needed for specific use cases
• Implement checks at the socket level to avoid TOCTOU (Time-of-Check-Time-of-Use) vulnerabilities

Describe alternatives you've considered

• Leave as-is with documentation only: Continue with just the warning in the README, but this leaves the reference implementation vulnerable and sets a poor security example

Additional context

• Consider adding example configurations to the Readme showing secure deployment patterns.