Allow metadata to be read from other projects

I would love to read metadata from multiple source folders when executing pipelines and workflows.
This would allow me to set up a lot of smaller projects which all inherit metadata from a single place.
It avoid having duplicated metadata all over the place for things like the local run configurations, data sets and so on.

Especially for integration testing this makes a lot of sense.

Right now metadata is sourced from a single metadata/ folder.

This easiest way to implement this could be on the level of the project.  I think we can just allow a list of projects to be specified to inherit variables and metadata from.