RepositorySimulator.add_key() only works for top level roles

[jku]

def add_key(self, role: str)

this assumes the delegating role is root. That's a fine default but I want to add keys for non-top-level roles too

This should work:
def add_key(self, role: str, delegator: str | None)

(and then use Root.type as the real default value)

[jku]

this is fixed in #129