Restore support for running custom events in Scenario

[tonyandrewmeyer]

In Scenario 6, it's possible to run custom events:

ctx = Context(...)
state_in = scenario.State()
state_out = ctx.run("my_charm_lib.on.foo", state_in)
assert state_out...

In Scenario 7.0, this has been removed: firstly because we got rid of the 'stringly-typed' events, and, more significantly, because we felt that the level of abstraction for Scenario is at the "Juju event". The intention was that people would run the Juju event that would cause the custom event to be emitted, e.g. ctx.on.relation_changed() might end up causing a DatabaseCreated event to be emitted.

@gruyaume pushed back on this:

our charm unit tests shouldn't know about library specificities. Our charm does not look into the relation databags, it only interfaces with the library. Taking the approach you are proposing entails our charm unit test to replicate the behavior of libraries that we don't have ownership over.

I feel like the charm author probably does need to know which Juju events will cause the custom events to be able to manually use the charm and to be able to write integration tests, and that you shouldn't need to manage the relation databags.

However, there are potentially many events and you have to know which ones might be triggered for what's happening. For example, with the data_platform DatabaseRequires class from data-platform-libs, it probably needs something like:

def test_credentials():
    ctx = Context(DemoAPICharm)
    db_rel = Relation("database")
    container = Container("webapp", can_connect=True)
    state = State(relations={db_rel})
    state = ctx.run(ctx.on.relation_created(), state)
    state = ctx.run(ctx.on.relation_joined(), state)
    state = ctx.run(ctx.on.relation_changed(), state)
    state = ctx.run(ctx.on.secret_changed(), state)

    assert state.get_container("webapp")... # check that the config file is there or whatever

Maybe tests that rely on custom events should be Catan tests? I think what I want to be testing is that when I integrate the two charms the state of the charm I'm writing ends up the way I expect.

However, maybe we should add back some way to run custom events explicitly. @gruyaume also needed the ability to provide custom arguments for creating the event, which Scenario 6 can't do.

Maybe this looks like this?

ctx = Context(...)
state_in = State(...)
state_out = ctx.run(ctx.on.custom_event('database.on.database_ready', x, y=42), state_in)
assert state_out...

custom_event would be a static method on CharmEvents with a signature like (attribute: str, *args, **kwargs) and it would pass *args and **kwargs through to the custom event class.

Moved from canonical/ops-scenario#177