Support S/MIME email signing

### Code of Conduct

- [x] I agree to follow Django's Code of Conduct

### Feature Description

I would like to see a new parameter in settings.py
`SIGNING_EMAIL_CERTIFICATE = /path/to/certificate.p12`
This will allow emails that are being sent by the app to be signed, if the sender email matches the email contained in the certificate.

### Problem

This feature will allow the users that are receiving the email to confirm that the email comes from the app which also increase trust in the app.

### Request or proposal

proposal

### Additional Details

_No response_

### Implementation Suggestions

_No response_

Start with settings.py and Django's email-sending path, then research how S/MIME signing should consume a .p12 certificate and match the sender address. The work is done when the proposed setting produces signed emails only for matching senders, with the behavior and coverage agreed for the feature.

Written by the indexing model from the issue text.