[Feature] Add neutral/informational message type or ability to customize message/result

[khitrenovich]

We are using this action in many workflows to notify about success/failure, and it works reliably.

Now we want to add a notification about a pending workflow that requires manual approval - and it looks like this action will always append a job status to the message (https://github.com/lazy-actions/slatify/blob/master/src/slack.ts#L88).

I suggest to either add info message type with neutral color and no status appended, or an ability to completely customize both the color and the message. This will be very helpful.

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