repository: maybe provide a Repository.get_delegating_role()

[jku]

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

[jku]

Oh this is going to be a nightmare if we allow "diamond" delegation structure (mutliple roles in different delegating metadata delegating to same name)

[lukpueh]

cc #660