Show the type of executor each shared Runner is configured with

Following the comment at https://gitlab.com/gitlab-org/gitlab-ce/issues/14732#note_4501705, it would be cool to show the type of the executor each Runner is configured with.

See also: #299758[View_specific_runner.png] (comment 728646016)

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/...>

cc @ayufan @tmaczukin