feat: add GitHub env var to support Enterprise onprem

[moosh3]

Description

This tweak allows the user to pass a custom URL for the GitHub instance, supporting onprem enterprise instances.

Server Details

• Server: github
• Changes to: resources

Motivation and Context

Currently this server is incompatible with users that work with GitHub Enterprise onprem. Swapping the API url as a variable allows for values such as "git.mycompany.com/api/v3/"

How Has This Been Tested?

Tested with goose

Breaking Changes

N/A

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