Add support for Slack apps instead of (or in addition to) "Incoming Webhooks"

[bclarkejr]

Slack has deprecated the "Incoming Webhooks" feature and is now encouraging everyone to switch to using Slack apps instead. Details are mentioned in the description here: https://slack.com/apps/A0F7XDUAZ-incoming-webhooks

Here is a simple example I wrote to post messages using a Slack app and the Slack Rest API:

import os
import urllib3
 
http = urllib3.PoolManager()
# Note you will need to grab this and set it as a system environment variable
# Or, presumably, the payload coming from Splunk
token = os.environ['SLACK_BOT_TOKEN']
 
r = http.request(
    'POST',
    'https://slack.com/api/chat.postMessage',
    headers = { 'Content-Type': 'application/json', 'Authorization': f'Bearer {token}' },
    body = '{ "channel":"#general", "text":":slack: Web API test." }'
)
 
print(str(r.read()))

Here are the steps that I think will be needed to support this:

1. Add a new configuration option to the alert action called SLACK_APP_OAUTH_TOKEN or something similar.
2. Update the code to branch on whether a webhook or oauth token is provided. Or potentially release a new major version that breaks the incoming webhook configuration.
3. Update logic related to using a Slack app.
4. Update documentation to point to Slack app documentation.

If this isn't completed in the next few months (which is our timeline for switching to Slack apps), then I will come back and update myself. However, I'm filing this request early to see if anyone else wants to pick it up before I can.

Let me know if you need any more information.

Thanks!