Start capturing "executor" from gitlab-runner

Summary

In order to display and filter runner information about which executor is running, we should start capturing this in the GitLab Rails application. In #299758[View_specific_runner.png] (comment 728646016) @tmaczukin said.

What we're missing is to catch and store this within the Runner object on the Rails side.

Proposal

Add a new column to the ci_runner table to capture this data. And capture this data when runners <register/contact instance/...>