Add Issue Milestones functionality

[jordanpartridge]

Summary

Add support for GitHub Issue Milestones to enable project planning and tracking capabilities.

Features to Implement

Milestone Operations

• List milestones for a repository
• Get a specific milestone
• Create new milestones
• Update existing milestones
• Delete milestones

Milestone Assignment

• Assign issues to milestones
• Remove issues from milestones
• List issues in a milestone

GitHub API Endpoints

• GET /repos/{owner}/{repo}/milestones
• GET /repos/{owner}/{repo}/milestones/{milestone_number}
• POST /repos/{owner}/{repo}/milestones
• PATCH /repos/{owner}/{repo}/milestones/{milestone_number}
• DELETE /repos/{owner}/{repo}/milestones/{milestone_number}

Priority

Medium - Important for project management