S/MIME: Automatically import sender certificates

Is your feature request related to a problem? Please describe.

Currently, it is cumbersome to import sender certificates due to two reasons:

1. It is not really possible to extract the certificate from an incoming email.
2. There is a separate modal in which I have to upload the certificate. So I need to download/extract the certificate, navigate to the modal, upload it again to import it.

Describe the solution you'd like

Certificats from emails with valid signatures should be imported automatically.

There are two possibilities here:

1. Import them automatically when the email is synced (after the signature verification).
2. Import them immediately when a user opens the composer to reply. The certificate needs to be imported in order to reply to an encrypted email. Hence, this is a nice opportunity to only import on-demand to prevent certificate spam.

Describe alternatives you've considered

No response

Additional context

Extracted from https://github.com/nextcloud/mail/issues/11569

Review the email sync and signature-verification flow, then trace the composer path used to reply to encrypted messages. Resolve whether import occurs after sync or on reply, and consider the work done when certificates from valid signed emails are imported automatically and support encrypted replies.