support for icon_url slack api parameter

### What feature do you want to see added?

Slack API allows to define either `icon_emoji` or `icon_url`

`icon_url` slack API parameter is not available in slack-plugin

### Upstream changes

_No response_

Start by locating how the slack-plugin handles the existing icon_emoji parameter and constructs Slack notification requests. Add support for icon_url alongside icon_emoji, then verify that the selected icon value is sent to Slack without breaking the existing option.

Written by the indexing model from the issue text.