Prevent wrong codeflow subscription configurations

[premun]

Context

Currently, it's possible to set the VMR codeflow subscriptions up incorrectly easily.
We need to prevent that by performing extra checks and rejecting some subscription creations/updates.

Goals

• Implement checks both on service side in the SubscriptionsController as well as in darc commands add-subscription and update-subscription.
• The checks apply only for codeflow subscriptions (source-enabled=true)
• Implement the following checks:
  • Backflow subscriptions (source directory not empty and source-enable is true): there cannot be 2 different backflow subscriptions defined to flow into the same branch of the same repository.
  • Forward flow subscriptions (target directory not empty and source-enable is true): there cannot be 2 different forward flow subscriptions defined to flow into the same VMR branch and same target directory.