Feature request: Include custom context in Email notification body

### Problem Statement

Currently, users cannot customize the email templates for Sentry notifications. This limitation makes it difficult to tailor the content to include relevant details.

Start by locating the email notification template and the existing notification context. Define how users will provide custom context and what the rendered email should contain, then identify the tests covering email notifications. Done means the customization behavior is specified, implemented, and covered by tests.

Written by the indexing model from the issue text.