Support Block Kit with Slack notifications

Hi Prometheus

Currently <slack-config> is using Simple formatting.

This is marked by Slack themselves as deprecated (or at least most hints dropped say: Format using Block Kit).

The new way to go seems to be Block-Kit.

I don't want to ask "Is there a plan?" as this would be a question. I'd like to pose as feature request to add something like:

<slack_config_blocks>
which would then accept stuff like:

And configuration like so (I am new to Yaml and Go Tmpl):

blocks:
- type: section
  text:
    type: mrkdwn
    text: ":fire: `Critical` alert in *production*: `ELK running out of shards (2
      left)`"
- type: section
  fields:
  - type: mrkdwn
    text: |-
      *Value before:*
      22
  - type: mrkdwn
    text: |-
      *Value now:*
      2
  - type: mrkdwn
    text: |-
      *Action:*
      Contact operations.
  - type: mrkdwn
    text: |-
      *Outcome:*
      X will happen.
- type: actions
  elements:
  - type: button
    text:
      type: plain_text
      emoji: true
      text: Victorops
    style: primary
    value: click_me_123
  - type: button
    text:
      type: plain_text
      emoji: true
      text: Silence
    style: danger
    value: click_me_123

Obvisouly my hardcoded strings would be replaced by {{ .alertProperties }}

Adding this I believe would make our DevOps plans (and hopefully others) dreams come true ❤️