[CI] Add bot action to auto-respond to first-timers asking "assign to me / can I work on this"

[chalin]

For example, see this comment originally posted by @svrnm in #8105:

... sure, give it a try, note that we are currently not assigning issues, so please provide a draft PR early

It would be nice to have a bot action to auto-respond to such inquiries. I think I've seen this sort of bot action in another CNCF-project repo.

A canned response might be something like:

Thank you for your interest in contributing to this project. If this is your first time contributing, then carefully read through Fixing an existing issue. As noted there, we don't assign issues unless you're a regular contributor.