Store channel/subscription configuration as code

[premun]

Context

There is no auditing, no change review process or no other control when devs are managing Maestro channels, default channels and subscriptions. This leads to user errors, unclarities and chaotic situations where context is often lost. Operations such as preview branch snap could also have more eyes on it (now also when we set up codeflow PRs) and the tooling for branch snap could be simpler.

It would be great if we had a more thorough system around the channel/subscription management:

• Entities could be in a repositories stored as configuration code.
• PRs would be needed to change the data.
• The service's configuration would reflect the contents of the repository.
• darc management commands would send users to this new flow (or open PRs for them).

Hackathon

We managed to do this during a hackathon. There is almost all of it implemented here.