[Feature]: Add a field for a secret to the Sink Type

[phillipahereza]

PR is the same as #313 that was accidentally deleted 🤦

Closes #309

📑 Description

This change allows developers to configure the sink with secrets. Additionally, it allows developers to create sinks where the authentication is separate from the endpoint.

apiVersion: core.k8sgpt.ai/v1alpha1
kind: K8sGPT
metadata:
  name: k8sgpt
  namespace: gpt
spec:
  ai:
    enabled: true
    backend: noopai
  sink:
    type: slack
    secret:
      name: slack-webhook
      key: url

✅ Checks

• [x] My pull request adheres to the code style of this project
• [ ] My code requires changes to the documentation
• [ ] I have updated the documentation as required
• [x] All the tests have passed

ℹ Additional Information

This PR breaks/affects #303