Add support for Slack Block Kit

[johanfleury]

Implements #2217

This is a draft PR in which I started implementing support for Slack Block Kit. It’s still missing a lot (from actual implementation to unit and acceptance tests), but I think it’s a good first step.

I’m still willing to work on this but I find it a bit messy and I’d rather have a first review/validation from project maintainers especially concerning:

• what to do regarding the inability for encoding/json to marshal map[interfaces{}]interfaces{} (see comment on SlackBlock.Config at line 407 in config/notifiers.go) and
• how to iterate with at lines 151 and 170 in notify/slack/slack.go (my first idea was to use reflect to find string values and apply the templating function on them).

Discussion on a “design” have taken place in #2217 and this might not be the best way to implement this feature. Let me know.

[carlosjgp]

Looking forward to see this implemented

[johanfleury]

Closing this as I don’t have time to work on it.