Spec file inheritance/inclusion

Describe the feature

In order to have a cleaner permifrost.yml file, we implemented the possibility of including (or inheriting) spec files.

Describe alternatives you've considered

Additional context

This isn't database specific, it's at spec loading (YML) level

Who will this benefit?

Any Permifrost user with large spec files

Are you interested in contributing this feature?

!146 (merged)