Enable alerting for FluxInstance events

[stefanprodan]

This PR integrates Flux Operator with notification-controller. Users can define Flux Alerts and get notified when the operator emits events during the FluxInstance reconciliation.

Setup alerting

Define an Alert for FluxInstance events:

apiVersion: notification.toolkit.fluxcd.io/v1beta3
kind: Provider
metadata:
  name: slack-bot
  namespace: flux-system
spec:
  type: slack
  channel: general
  address: https://slack.com/api/chat.postMessage
  secretRef:
    name: slack-bot-token
---
apiVersion: notification.toolkit.fluxcd.io/v1beta3
kind: Alert
metadata:
  name: slack-alert
  namespace: flux-system
spec:
  providerRef:
    name: slack-bot
  eventSeverity: info
  eventSources:
    - kind: FluxInstance
      name: flux

Notifications examples

The operator will forward events to notification-controller after the install operation succeed, after a successful update (version bump, config change, CVE patch, etc) or when the reconciliation fails.

[matheuscscp]

LGTM 👌