S/MIME signed/encrypted email support

[apparition47]

Not compatible with Sonoma Mail digital signature and encryption feature, which I use with the apporpriate certificates.

Expected:

Actual:

When viewing signed emails, the Signed label isn't displayed with the MailTrackerBlocker message security extension. Also the signing/encryption icons are greyed out in the message composer. Tested with Actalis' free S/MIME cert.

Content-Type: multipart/signed;
	boundary="Apple-Mail=_CB35ED7D-E91E-4556-85DF-1AFDCCA86A12";
	protocol="application/pkcs7-signature";
	micalg=sha-256
Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3774.600.62\))
Subject: signed test

--Apple-Mail=_CB35ED7D-E91E-4556-85DF-1AFDCCA86A12
Content-Disposition: attachment;
	filename=smime.p7s
Content-Type: application/pkcs7-signature;
	name=smime.p7s
Content-Transfer-Encoding: base64

[apparition47]

Implemented the "Signed" indicator when viewing messages in 1.1.6.

Composing emails with signing and/or encryption is yet to be implemented. For now, you'll need to disable the MailTrackerBlocker extension if you need to send signed or encrypted emails.