[VR-10494] Notification Channel CRUD

[convoliution]

from verta import Client
from verta.operations.monitoring.notification_channel import SlackNotificationChannel

client = Client().operations
channel = client.notification_channels.create(
    "slack-webhook",
    SlackNotificationChannel("https://hooks.slack.com/..."),
)

Changes

Config classes

• Add _NotificationChannel config classes (e.g. SlackNotificationChannel(webhook_url)) to verta.operations.monitoring.notification_channel

Entities

• Add NotificationChannel create & get via client.notification_channels.create(), etc.

[daniel-verta]

LGTM minus absence of integration tests.