Implement regex exclusions for alerts

[stefanprodan]

This PR adds a option field called exclusionList to the Alert API.

Fix: #137

Example usage

Skip alerting if the message matches a golang regex from the exclusion list:

apiVersion: notification.toolkit.fluxcd.io/v1beta1
kind: Alert
metadata:
  name: flux-system
  namespace: flux-system
spec:
  providerRef: 
    name: on-call-slack
  eventSeverity: error
  eventSources:
    - kind: GitRepository
      name: flux-system
  exclusionList:
    - "waiting.*socket"

The above definition will not send alerts for transient Git clone errors like:

unable to clone 'ssh://git@ssh.dev.azure.com/v3/...', error: SSH could not read data: Error waiting on socket