Add support for http(s) proxy when sending alerts

[stefanprodan]

Changes:

• Add Proxy optional field to Provider API
• Add URL prefix validation to CRDs
• Implement http(s) proxy for alerts

Proxy example:

apiVersion: notification.toolkit.fluxcd.io/v1beta1
kind: Provider
metadata:
  name: slack
  namespace: default
spec:
  type: slack
  channel: general
  address: https://hooks.slack.com/services/YOUR/SLACK/WEBHOOK
  proxy: https://proxy.corp:8080

Fix: #61

[hiddeco]

Given the existence of some other (low level) proxy protocols than HTTP/S (e.g. SOCKS5), I would prefix the field names with the protocol to guarantee we are not shooting ourselves in the foot right now, and leave room for other implementations later (if any).

[hiddeco]

On second thoughts, we can require a protocol to be defined and make decisions based on that, if it ever gets to supporting other things.