[Feature] TTL Health Check based process monitoring

Among our jobs there are tasks that run processes that cannot be monitored via health-checks like HTTP or TCP - usually it applies to async workers that process some tasks. There is no really good way to figure out how the process is doing but so far we've been monitoring whether process is alive with `ianitor` or `consul-announcer`. While migrating to Nomad we were able to allow nomad to take care of all health checks except this one. We still have to wrap our process into some "monitor" that will use TTL-based health-check to monitor the process health. I'm wondering whether such a health-check can be added to Nomad.

Start by locating Nomad’s existing health-check implementation and job configuration entry points. Compare the requested TTL-based process monitoring with the current HTTP and TCP checks and the Consul TTL approach described in the issue. Done should be a decided, specified way for Nomad to monitor process liveness through a TTL health check.

Written by the indexing model from the issue text.