Implement 3-way merge for dependency updates during backflow

[premun]

Context

The updates to the version files are a complicated problem because not only they can happen in the repo/VMR but a third source of truth are the backflow updates which bring in the asset versions that were just built.

During #4196, we implemented a naive strategy for conflict resolution of version files during backflows.
This strategy takes the versions from the target branch and updates it with current build's assets.
This means that any changes to the version files done in the VMR will be lost.
This is temporarily fine because the VMR is closed for changes and no changes to these file will occur.
However, we will need a proper solution so that changes can be done in either side.

Goal

• Implement a better conflict resolution strategy which will understand the changes since the last sync that happened in the repo, the VMR and which ones are updated by the currently flowed BAR build.
• Create test cases which will show various scenarios that can happen (dependencies being add/removed on one side or another). The base scenario to start from is the BackwardFlowConflictResolutionTest which describes the problem well.