Create a design for build-less forward code flow

## Context

The code flow model for code flowing from product repos to the VMR after the first Unified Build deliverable (.NET 10 timeframe) will end up looking like this:
1. Product repo merges a PR (in GitHub)
1. Commit is mirrored to Azure DevOps repo
1. Official build of the repo is started
1. **Repository might be built (to verify the commit is fine) but not necessarily as packages will be built in the VMR**
1. Build is registered in BAR and assigned to a channel
1. Code flow subscription is triggered from the channel to VMR and PR with code changes is opened in the VMR

Technically, step `4.` can be a very quick operation - a no-op build that just needs to ping Maestro. But it still requires YAML, some tooling, an agent to boot etc. Maybe we could skip this step?

## Goal

- Explore how we could trigger forward flow updates without the need to queue an official build.
Example: Maestro watches for push commit events and acts on these directly. Or maybe the mirroring service pings Maestro.
- Create a design with a proposal for a possible north-star the code flow should aim for.
The design should also mention possible intermediate milestones and how to reach the end state.