Allow "Additional data" (extra) to be displayed in Slack integration notification

### Problem Statement

I want to be able to send additional data from an event and have it showing up in Slack directly, for quicker triages.

Example:
```
Sentry.captureMessage('Test 1', {
extra: {
test: 'test',
now: Date.now(),
now_iso: new Date(),
},
});
```

Which displays in Sentry here:

But nothing appears in Slack, I even tried to set the tags option in the alerts notification to include `extra`, but I think it's considered something else and not really a tag internally.

### Solution Brainstorm

It would be nice to have an option where it would be displayed, even if it's under a 3-backtick markdown block.

### Product Area

Alerts