email: enable `email-version` field in header to override Connect version

For testing within dev or other versions of Connect, it would be nice to have access to a hook to force a V2 email format. This was mentioned in #13882

Setting it as follows is the intention

```
---
title: Email Test Document
author: Jules
format: 
  email:
    email-version: 2
---
```

This way, even if we detect `SPARK_CONNECT_USER_AGENT < 2026.x` we can still output the V2 format.
