repository: maybe provide a Repository.get_delegating_role()

I wonder if Repository.get_delegating_role(role: str) -> str would be useful

• In my implementations I end up looking up the delegating role of a role quite a lot.
• if this existed, Repository could also offer QOL things like Repository.get_verification_result(role, md)

The reason this does not exist is that we can't really provide a fully generic implementation: Repository does not maintain a delegation tree. So we could

• Require all Repository implementations to provide the method OR
• Provide a default implementation that assumes max one level of targets delegations (and require more complex implementations to provide their own) OR
• Provide a default implementation that adds a delegation tree into Repository

Start by reviewing the Repository API and its implementations to see how delegating roles are currently looked up and how delegation trees are represented. The main task is to resolve whether Repository should require the method, provide a one-level default, or maintain a delegation tree; done requires an agreed API and behavior for the proposed role lookup and related quality-of-life methods.