API for Ruby defined Services

What does this Merge Request do and why?

Cherry-picks from !1197 (merged) and improves on basic API for defining Services in GDK outside of a Procfiles.

This includes a refactor on runit/config extracting all the templates to their own separate template file and some minor improvements

This also includes a fallback to use legacy service whenever a service has not been migrated yet, so GDK can be fully functional with the experimental flag enabled.

Related to #904

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.