### What
This PR changes the slack-app addon to use slack-api's scheduleMessage
instead of postMessage.
### Why
When using postMessage we had to find the channel id in order to be able
to post the message to the channel. scheduleMessage allows using the
channel name instead of the id, which saves the entire struggle of
finding the channel name. It did mean that we had to move to defining
blocks of content instead of the easier formatting we did with
postMessage.
### Message look
We've been struggling with not getting hold of all channels. Reading the
documentation I found the support for next_cursor in the
response_metadata, as long as the response has a next_cursor, there are
responses left, so this PR adds a loop for querying until the next
cursor or the channels list is undefined or `''`. This should solve the
issue Wayfair had as well.
https://linear.app/unleash/issue/2-1232/implement-first-iteration-of-the-new-slack-app-addon
This PR implements the first iteration of the new Slack App addon.
Unlike the old Slack addon, this one uses a Slack App (bot) that is
installed to Slack workspaces in order to post messages. This uses
`@slack/web-api`, which internally uses the latest Slack API endpoints
like `postMessage`.
This is currently behind a flag: `slackAppAddon`.
The current flow is that the Unleash Slack App is installed from
whatever source:
- Unleash addons page;
- Direct link;
- https://unleash-slack-app.vercel.app/ (temporary URL);
- Slack App Directory (in the future);
- Etc;
After installed, we resolve the authorization to an `access_token` that
the user can paste into the Unleash Slack App addon configuration form.
https://github.com/Unleash/unleash/assets/14320932/6a6621b9-5b8a-4921-a279-30668be6d46c
Co-authored by: @daveleek
---------
Co-authored-by: David Leek <david@getunleash.io>
