Update to Slack's "Block Kit" API from the old "Attachment" API

[brianespinosa]

Describe your enhancement idea

Slack released their preferred "Block Kit" API in 2019 which gives more control over message formatting over the legacy "attachment" API. It would be beneficial if we updated this plugin for support for the newer, preferred block kit API to allow creating more useful notifications.

If we wanted to maintain backwards compatibility with existing fields, there is a guide that would be helpful here.

Example Use Case
I generate a report that I want to alert my Slack channel on a set schedule. On this report I might want to set the border color of a field section to red if my calculated AVAILABILITY number is below a set threshold.