Support for Slack Bot (App) Token

[sammcj]

It would be great if this could use a standard Slack App Bot User OAuth Token rather than passing a webhook URL.

This would allow the bot to post across several rooms and users and allow people have the legacy webhooks disabled to use this Action.

[jprecuch]

I would love this feature implemented as well. Same like https://github.com/slackapi/slack-github-action has