Support for non-linear DAGs

[RobbeSneyders]

We currently only support non-linear DAGs, which limits the expressiveness of Fondant pipelines.

To support non-linear DAGs, we would need at least the following functionality:

• Compiling to non-linear DAGs in the underlying runners
• The ability to merge manifests from multiple branches to one

Especially the last one is complex.

• We probably cannot assume that the partitions will be aligned
• Which merge strategy to apply? Inner, outer, left join?

[PhilippeMoussalli]

I feel like this feature is needed but would not really prioritize it at the moment unless it's currently blocking us/other users from implementing a given pipeline