allow optional expiration time during setup

[tomholub]

first should implement #942

I took issue with FlowCrypt for two things that could not be provided through the browser extension.

1. You could not specify an expiration date for your certificates.
2. You could not generate revocation certs.

[tomholub]

Moving to Not anytime soon our handling of expired prvs is not good yet. Would first have to have a way to renew them later when they expire.

[seisvelas]

ref: https://mail.google.com/mail/u/human@flowcrypt.com/#inbox/FMfcgxwLsmclfNqschwFSkCcGcMHSmJF