Integrate Maestro++ with .NET Product Construction Service

[premun]

Context

As part of https://github.com/dotnet/arcade-services/issues/2994, a new service .NET Product Construction Service will be created. This new service will be called by and will call the Maestro++ service. The details are described here (Full Code Flow.md).

Goal

We need to make the appropriate changes to the Maestro++ service to handle code-flow-enabled subscriptions and exercise this new flow of preparing a PR from a branch that will be asynchronously created by the .NET Product Construction Service service.

Maestro++ needs to call .NET Product Construction Service where appropriate but also understand a new state of subscription/PR when it's waiting for .NET Product Construction Service.

Ideally, we have an E2E scenario tests, similarly to what we have for other flows.

Work Items

• [x] #3320
• [x] #3316
• [x] https://github.com/dotnet/arcade-services/issues/3368
• [x] #3356
• [ ] #3317
• [ ] #3302
• [ ] #3318
• [ ] #3319
• [ ] #3352
• [ ] #3467

[mmitche]

Costing in https://github.com/dotnet/arcade-services/issues/2994