Extend Maestro++ subscription model with code flow capabilities

[premun]

Context

For flowing code between the VMR and the individual repositories, we will utilize the same mechanism as for current dependency flow - builds, channels and subscriptions.
.NET developers will create subscriptions between the repos and the VMR of a special kind denoting that code will be flown too and these will then be triggered/processed by the Maestro++ service which will in turn call the .NET Product Construction Service.

Goal

To get the above working, we need to extend the current model with the new code flow metadata and teach darc to manage these new code flow subscriptions.

Work Items

• Implement backflow of a build (code + dependencies) #3252
• Implement forward flow of a build (code + dependencies) #3253
• Extend the BAR model with codeflow metadata #3254
• Add codeflow subscription management darc commands #3255

[premun]

The model has been extended. The last column will be rolled out in 2 days