feat(slack): optional blokkit payload for Slack notifications

[TheMeier]

Pull Request Checklist

Please check all the applicable boxes.

• Please list all open issue(s) discussed with maintainers related to this change
  • Fixes Support Block Kit with Slack notifications #2217
• Is this a new Receiver integration?
  • I have already tried to use the Webhook Receiver Integration and 3rd party integrations before adding this new Receiver Integration
• Is this a bugfix?
  • I have added tests that can reproduce the bug which pass with this bugfix applied
• Is this a new feature?
  • I have added tests that test the new feature's functionality
• Does this change affect performance?
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  • I have added new benchmarks if required or requested by maintainers
• Is this a breaking change?
  • My changes do not break the existing cluster messages
  • My changes do not break the existing api
• I have added/updated the required documentation
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• I will follow best practices for contributing to this project

Which user-facing changes does this PR introduce?

ENHANCEMENT SLACK: Add optional, configurable block-kit payload

[TheMeier]

This is the approach to acheive option 2 from this comment #2217 (comment) using other means. It relies on template.DeepCopyWithTemplate which is problematic, but the situation with that can maybe be improved in the future.