Implement support for codeflow subscriptions

[premun]

Context

As part of #2994, a new service .NET Product Construction Service will be created. It will be processing the new codeflow-enabled subscriptions.
The details are described here (Full Code Flow.md).

Goals

• We need 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.
• We have an E2E scenario tests, similarly to what we have for other flows.
• We handle all the non-happy paths and edge cases when things go wrong.

[mmitche]

Costing in #2994