Find a better solution than patching attributes onto nornir objects

[bytinbit]

We patch attributes onto existing objects at two locations in the code:

• napalm_ping.py: we add a destination field to every Nornir-Result.
• result.py: we add an exception field to a nornir_result

There should be a more elegant solution to this - currently, mypy ignores this error using # type: ignore[attr-defined].