feat: add mattermost sink

[tozastation]

Closes #300

📑 Description

1. added mattermost sink
   • support mattermost incoming webhook
2. added parameter on sink (K8sGPT API)
   • Channel (Mattermost Optional Param)
   • Username (Mattermost Optional Param)
   • IconURL (Mattermost Optional Param)
   • Mattermost Docs

✅ 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

1. Confirmed that no value is entered in the notification content when ai is set to enable: false. (current Slack notifications may be notified with no value as well.)

Change to -> I tried to load error text when ai is enable: false.

3. E2E Test Contents
   • test assets is here
   • deploy mattermost server
   • applying resources detected in k8sgpt
   • confirmation of incoming notifications

ai.enable: false pattern

ai.enable: true pattern

[tozastation]

@AlexsJones PR created. Please review when you are available...!

[arbreezy]

@tozastation you also have to manually update the helm chart crd template

[tozastation]

@AlexsJones @arbreezy Thanks for the review !! I will respond to it !!

[tozastation]

@AlexsJones

I added an explanation to README.md of k8sgpt-operator. Please check it...! Which file should I put in k8sgpt-ai/docs?