Alerts - Possibility to add additional data to the payload/json

### Problem Statement

In Splunk Observability Cloud, each detector have the possibility to customize the message that you send over the webhook if possible for more integrations. The Tip field becomes part of the json body sent.

So we need to have webhook integration where we can add the header, and per alert where we can add key-value pairs.

We want to be able to configure more on the webhook, just like some extra options for the Slack integration.

### Solution Brainstorm


### Product Area

Alerts

┆Issue is synchronized with this [Jira Improvement](https://getsentry.atlassian.net/browse/FEEDBACK-1907) by [Unito](https://www.unito.io)