Add gitaly RunIt service to GitLab Omnibus

DJ Mountney requested to merge add-gitaly-service into master

Closes: https://gitlab.com/gitlab-org/omnibus-gitlab/issues/1832

The package does not include the executable itself at this time.

We intend to initially ship it seperatly, to allow it to iterate in production on a seperate schedule from GitLab, at least initially.

But if you have the executable installed, and you enable the service, It should now be run using RunIt and available during gitlab-ctl status/up/down/restart

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