HTTP Proxy support for official Slack Alert Notification

## What?

Previously used plugin (https://github.com/graylog-labs/graylog-plugin-slack) supported setting HTTP Proxy so that Slack notifications can utilize using of HTTP Proxy. The current officially supported Slack Alert notification does not allow this to be configured (https://docs.graylog.org/docs/alerts).

## Why?
If a server is placed so, that outgoing traffic requires the use of HTTP Proxy, Slack notifications cannot be used with the official Slack Alert notification feature.

## Your Environment

* Graylog Version: 4.2.5
* Operating system: Centos 8