allow signing of outgoing s/mime emails

[tomholub]

dependent on #3530

[tomholub]

It would be great to do this one after #3530

However if other things should be done first, I'll leave it up to you - with the goal to do this issue whenever all issues leading up to it were settled. Thanks!

[hotzen]

this issue is still listed as being relevant for missing/limited S/MIME signing support here: https://flowcrypt.com/docs/technical/feature-availability.html

Is S/MIME signing still an issue?