Add Sidekiq health-check configuration keys

We are looking to separate health-check endpoints from metrics endpoints in Sidekiq, since the latter will move out into a separate server process, while health-checks should remain in the worker that is elected as cluster leader.

To that end we should introduce new configuration keys in Omnibus and Helm Charts that allow setting the health-check port, e.g. gitlab.monitoring.sidekiq_health_checks (it should mimic the layout of the sidekiq_exporter configuration, which we will continue to use for metrics endpoints.)

The new settings should default to the values (or defaults) for sidekiq_exporter.

Once we actually segregate those endpoints, Sidekiq should only consult this new setting to determine where to serve health-checks.