s2n should have a way to extract a certificate expiry timestamp

[fsareshwala]

Problem:

Currently there is no way to ask s2n for the expiry timestamp of a given certificate. Right now, if developers want the expiry time of a certificate, they have to code against openssl APIs directly. This is not fun. Instead,

Proposed Solution:

We should add a method to allow developers to get the certificate expiration time. We've needed this internally at Twitter and have implemented a solution against the openssl libraries. Happy to add that to s2n as well.