[Notifier] [Slack] Validate token syntax

[OskarStark]

Q	A
Branch?	5.x
Bug fix?	no
New feature?	yes
Deprecations?	no
Tickets	---
License	MIT
Doc PR	-

This PR follows #39560

@odolbeau @malteschlueter @norkunas @fabpot can you confirm all your tokens start with xox?

From the Slack documentation:

• Bot user token strings begin with xoxb-
• User token strings begin with xoxp-
• Workspace access token strings begin with xoxa-2

[derrabus]

The code looks correct, but I wonder if our code should have an opinion on valid Slack access tokens. To me, a token is a string that I present as I've received it if I need to authenticate. We usually don't need to parse such a token in any way. The format of those token is an implementation detail that Slack might decide to change anytime.

[OskarStark]

I agree but in this case Slack is dealing with webhook ids and tokens which can be confusing, I think it's much easier to adopt this bridge if you know you are in the right place or not.

[fabpot]

Thank you @OskarStark.