[Issue Alerts] Allow user details to be used as tags in Slack notifications

### Problem Statement

When creating/editing an **issue alert rule**, you can provide a comma-separated list of addition tags to append to a Slack notification.

I see two problems with this feature:
1. There are no clues indicating which tags can be entered. The input is a simple text field and does not offer any kind of autocomplete.
2. There are tags that don't seem to be valid here, for example user details.
- E.g., I wanted to append the email of the affected user to the Slack notification.
- I tried `user.email`, `user_email`, `email`, `User: Email`, `"User: Email"`.
- None of them worked.

### URL

```
https://.sentry.io/issues/alerts/rules///
```

### Screenshot

### Solution Brainstorm

1. Provide a list of available tags when the input is selected, for example via auto-completion, a drop-down or a multi-select list in a modal.
2. Add user details to the list of available tags.

### Product Area

Alerts

The issue-alert rule editor and Slack notification tag input are the named entry points; start by tracing that UI from the provided rules URL and checking how available tags are defined. Done means the editor clearly exposes valid tags, supports the requested user-detail values such as email, and the selected values appear in Slack notifications.

Written by the indexing model from the issue text.