Runner API add "last_job_at" field in "Get runner’s details"

Proposal

add a new field "last_job_at" in GET runner details api: https://docs.gitlab.com/ee/api/runners.html#get-runners-details

And go further

add a maintenance feature in Admin pages to define a runner cleanup policy, ex: cleanup runner with no job since 3 months

Background

We manage a gitlab instance, and our users have created lots of private runners, and they always forget to unregistrer them.

So we want to cleanup the abandonned private runners, now we use a script to check if the "contacted_at" is more than 3 months, then remove them.

But there are also active abandonned runners, contact gitlab but do nothing, is this case we want get the "last_job_at" information to cleanup for example, runner has no job since last 3 months, we clean it.

{
    ...
    "contacted_at": "2016-01-25T16:39:48.066Z",
    "last_job_at": "2016-01-24T16:39:48.066Z"
    ...
}

This will be really helpful in cleanup maintenance.