[Feature Request]: Slack Token

**Is your feature request related to a problem? Please describe.**
One thing that's quite frustrating (about Slack) is that even though you can sort of specify Slack channel names with a webhook URL, sometimes they get ignored. If your webhook comes from a modern Slack app (i.e. via the `incoming-webhook` scope in a Slack app), then the channel is “fixed” at the time the webhook is configured. You cannot override the channel at runtime by sending a channel field in the payload.

I used `AppRise` in `Uptime Kuma` to route notifications which wasn't the best.

**Describe the solution you'd like**
I think a really simple solution would just be to add a `Bot Token` field to the Slack provider, and use the token if it's set, with the channel.

**Describe alternatives you've considered**
AppRise, but it's not supported in Peekaping (yet)?

**Additional context**
N/A

Look for the Slack notification provider code in the project, likely in a directory like 'notifications' or 'providers'. Understand how webhook URLs are currently handled. Research the Slack API for sending messages with a bot token. The change involves adding a token field, modifying the payload construction, and updating the sending logic. Test by sending a notification to a Slack channel.

Escrito por el modelo de indexación a partir del texto del issue.