Remove object.__setattr__ frozen bypasses in Scenario

[tonyandrewmeyer]

I think we're all agreed that it would be nice if we didn't have to have a bunch of object.__setattr__(self, "attribute", new_value) in the state (while still keeping the immutability that the frozen dataclasses provide).

A few of these are in __post_init__ and probably need to stay as they are. The majority are handling components of the state being changed during the event - new secret content, opening/closing ports, setting the unit/app status, etc. It would be nice to keep the immutability but avoid the object.__setattr__ for those.

Moved from canonical/ops-scenario#170