Add FFI function to retrieve key validity time.

[ni4]

Description

Key/subkey validity time may depend on different signatures: certifications (and their revocations), direct-key signatures, key revocations, and so on.
This logic should be implemented into the single function, so user may understand whether he should believe, for instance, to the message, signed 2 years ago with the key which expired 1 year ago (or 3 years).

[mkmelin]

Any update on this? Would help with https://bugzilla.mozilla.org/show_bug.cgi?id=1673240

[ni4]

@mkmelin I started to work on this together with #1194 , so hopefully in a week or two it will be merged into master.