Replace custom events with gets/sets

[gruyaume]

Overview

As we are now using central hook event handlers, custom events don't make as much sense. Here the enhancement request is to replace the custom events with gets/sets.

Rationale

This simplifies the code quite a bit. Right now we have a complicated series of events where the charm library triggers a custom event that it does not know of. Making the change described here also makes it possible for the lib not to be a child of CharmBase.

Proposal

1. Replace custom events with configure() and is_configured() calls
2. Replace func type attributes with list of NAD's and list of Network Annotations
3. Make the library a root class (instead of having ChildBase as a parent). Pass in everything k8s related (statefulset name, namespace, etc.)

Current

class NadConfigChangedEvent(EventBase):
    """Event triggered when an existing network attachment definition is changed."""

class UpfOperatorCharmEvents(CharmEvents):
    """Kubernetes UPF operator charm events."""

    nad_config_changed = EventSource(NadConfigChangedEvent)

class UPFOperatorCharm(CharmBase):
    """Main class to describe juju event handling for the 5G UPF operator for K8s."""

    on = UpfOperatorCharmEvents()  # type: ignore

    def __init__(self, *args):
        super().__init__(*args)

        self._kubernetes_multus = KubernetesMultusCharmLib(
            charm=self,
            container_name=self._bessd_container_name,
            cap_net_admin=True,
            network_annotations_func=self._generate_network_annotations,
            network_attachment_definitions_func=self._network_attachment_definitions_from_config,
            refresh_event=self.on.nad_config_changed,
        )
        self.framework.observe(self.on.config_changed, self._on_config_changed)

    def _on_config_changed(self, event: EventBase):
        """Handle for config changed events."""
        ...
        self.on.nad_config_changed.emit()

Proposed

class UPFOperatorCharm(CharmBase):
    """Main class to describe juju event handling for the 5G UPF operator for K8s."""

    def __init__(self, *args):
        super().__init__(*args)

        self._kubernetes_multus = KubernetesMultusCharmLib(
            namespace=self.model.name,
            statefulset_name=self.model.app.name,
            container_name=self._bessd_container_name,
            cap_net_admin=True,
            network_annotations=self._generate_network_annotations(),
            network_attachment_definitions=self._network_attachment_definitions_from_config(),
            refresh_event=self.on.nad_config_changed,
        )
        self.framework.observe(self.on.config_changed, self._on_config_changed)

    def _on_config_changed(self, event: EventBase):
        """Handle for config changed events."""
        ...
        if not self._kubernetes_multus.is_configured():
            self._kubernetes_multus.configure()