Custom Spawn message framework

[rtmiz]

Add: add_custom_event method, custom_event_queue trait,
Update: events property to handle custom events

Adjust KubeSpawner class so you can easily include custom messages for the user during the spawning process, for instance like so (When using hooks inside of a jupyterhub config):

import time
from datetime import datetime, timezone

def modify_pod_hook(spawner, pod):
    spawner.add_custom_event(
        eventTime=str(datetime.now(timezone.utc)),
        lastTimestamp
        message="Test event. This is a test event inside of modify_pod_hook.",
        type="Custom",
        involvedObject: {"name": spawner.pod_name},
        metadata: {"uid": str(time.time())}
    )
    return pod

Closes #899

[rtmiz]

Seems like the failing tests are related to #895