Slack Provider: Fixes slack_username and adds slack_icon_emoji

[gabrielsoltz]

Hi, this PR fixes and implements the use of the slack_username configuration, which was previously non-functional.

I'm also adding slack_icon_emoji, which sets the emoji for the username. When adding this, I noticed that shoutrrr does not support setting icon_emoji in Slack messages.

Slack expects the icon_emoji to be included in the JSON payload of the POST request, but shoutrrr only supports basic message text and allows the username through a query parameter — it does not permit modifying the JSON payload structure to include icon_emoji.

To fully support both username and emoji, I replaced the shoutrrr.Send() call with a simple http.Post using a custom JSON payload. This provides complete control and ensures compatibility with Slack’s webhook API.

I hope this is okay. If not, I can leave the slack_icon_emoji out of this PR and revert to the shoutrrr implementation, but at least it will make the slack_username work.

Summary by CodeRabbit

• New Features

  • Added support for customizing the Slack message icon using an emoji when sending notifications.

• Documentation

  • Updated the example configuration in the README to show how to set a custom emoji icon for Slack notifications.