Export: Slack block JSON

[Jguer]

Export Slack block JSON

What

Implement support for Slack json block output.

https://app.slack.com/block-kit-builder/

Why

So it's easy to curl the output of ggshield directly to a slack and get a nice display

Command

ggshield scan --output-type=slack ci

ggshield scan --output-type=text ci

ggshield scan --output-type=json ci

Definition of Done

• [ ] Slack output handler
• [ ] --json ported to --output-type ( if not done already)