Add versions/milestone function.

[HasutoSasaki]

We are making good use of the MCP server!
In our work, there was a use case where we wanted to set milestones in MCP all at once.
Therefore, we would like to add the versions/milestone function.

Currently, milestone IDs can be referenced in issue operations, but there are no tools to manage milestones themselves. I'd like to implement:

• get_version_milestone_list: List versions/milestones for a project
• add_version_milestone: Create a new version/milestone
• update_version_milestone: Update existing version/milestone
• delete_version: Delete a version

If implementation has not yet progressed, I would like to proceed with implementation by creating a PR. Is that acceptable?

[trknhr]

@HasutoSasaki Thanks! Please feel free to proceed if you're able.

[HasutoSasaki]

@trknhr
Thank you for your review and other assistance!
Since you've merged the PR, I'll close the issue.
#16