[Feature]: Add exclude list to model reset

[sed-i]

Description

Currently, reset would entirely empty out a model.

For some integration tests it could be handy if the we could provide an include-list for removing (or exclude-list for retaining) apps and their relations.

Urgency

Casually wishlisting

Code I'd Like to Run

For example, given charms A -- B -- C -- D, it could be handy if

model.reset(retain=["B", "C"])

would leave us off with B -- C.