Make the time it takes runners to go offline configurable

Problem to solve

Runners will report as offline when inactive for 1 hour. This is currently hardcoded.

Proposal

Make ONLINE_CONTACT_TIMEOUT configurable.

What does success look like, and how can we measure that?

The ONLINE_CONTACT_TIMEOUT is configurable to allow administrators to determine the amount of time it takes an inactive runner to go offline.

Links / references

https://gitlab.com/gitlab-org/gitlab/blob/master/app/models/ci/runner.rb#L25

Customer ticket: https://gitlab.zendesk.com/agent/tickets/139413 (internal use)