Add alert summary to notification metadata

[stefanprodan]

Changes:

• Add summary (optional field) to Alert API
• Add alert summary to notification metadata

Example:

apiVersion: notification.toolkit.fluxcd.io/v1beta1
kind: Alert
metadata:
  name: ingress
  namespace: nginx
spec:
  summary: "Ingress traffic affected in production (us-west-2)"
  providerRef: 
    name: on-call-slack
  eventSeverity: error
  eventSources:
    - kind: HelmRelease
      name: nginx-ingress

Fix: #94

[phillebaba]

LGTM