Add token expiration to runner details in API

What does this MR do and why?

This MR will add token_expires_at details in the Runners API responses that show Runner details.

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

I've confirmed that this works as expected in my local GDK on the runners/all and runners/ endpoints.

Screenshots are required for UI changes, and strongly recommended for all other merge requests.