Allow selecting built-in tags in Event Highlights modal on the Issues page

### Problem Statement

When configuring the Event Highlights section on the Issues page of the Sentry Web UI, the options available are quite limited. Currently, users can only choose from a small set of Tags and Context values.

However, some very useful built-in tags are not exposed here. For example:

The http.referer tag cannot be added as an event highlight. More generally, any HTTP Request headers are not available for highlighting.

### Solution Brainstorm

_No response_

### Product Area

Issues

Start by locating the Event Highlights modal on the Issues page and trace how its current Tags and Context options are populated. Inspect how built-in tags and HTTP Request headers are represented and determine how they can be exposed for selection. Done means users can select http.referer and other HTTP Request headers as event highlights.

Written by the indexing model from the issue text.