Create tooling for redirecting of subscriptions

[premun]

Context

Based on the outcomes of #4362, we will need a tool that will update/disable/add subscriptions for a given repo to move it onto the flat flow.

Goal

The exact functionality will be added but roughly it will need to be pointed to a repo that is inside the VMR and change the subscriptions around its default branch:

• Load all outgoing subscriptions (matching this repo and the default channel for the default branch of this repo), change their source to the VMR as they will be getting their packages from there. (does not apply to Arcade).
• Create code flow subscriptions between the repo and the VMR.

It is still an open question whether it should not also do some first backflow (or just prepopulate the repo as there was one.