Feature Request: Enable UTM Parameter Support for Links in Jetpack Newsletter Emails

### Impacted plugin

Jetpack

### What

I would like the ability to automatically append UTM parameters (e.g., utm_source=jetpack-newsletters, utm_campaign=post_notifications) to links included in Jetpack’s post notification emails. This would allow traffic from these emails to be accurately tracked in the UTM section of Jetpack Stats alongside other sources.

Currently, post notification emails do not support UTM tagging, so this traffic is not represented in Jetpack Stats’ UTM tracking views (Source/Medium/Campaign tabs), making it difficult to evaluate the impact of these emails in the context of other marketing efforts.

### How

- Add a setting in the Jetpack Newsletter settings to enable UTM tagging for links in outgoing notification emails.
- The toggle could append default tags like utm_source=jetpack-newsletters and utm_campaign=post_notifications.
- Ensure the UTM parameters persist on all post permalinks within the email body (and optionally for comment/title/CTA links).
- These tagged visits should then appear in the UTM report under Jetpack → Stats → Traffic → UTM.

Optional: A customizable field for users to define the UTM value, i.e., jetpack-newsletters and the likes...

No file or test is named. Start by locating the Jetpack Newsletter settings and the code that builds post notification email links, then trace how Jetpack Stats records UTM visits. Done means an opt-in setting applies the agreed UTM parameters to relevant email links and tagged visits appear in the UTM report.

Written by the indexing model from the issue text.