Add a tool to assign copilot to issues

[williammartin]

Describe the feature or problem you’d like to solve

With it becoming possible to assign Copilot to issues, it would be nice to add a tool to allow this, in the same way copilot can be requested for reviews

What Prompt would I expect to use this tool

Please assign copilot to

[aryasoni98]

@williammartin Would you like me to also help you draft a PR or suggest how to implement it if you're planning to code it out?

[williammartin]

There's already a draft PR #399 thanks.