Retrieving valid_from/valid_until from a certificate

[abbysmal]

Hello,

I currently need to get the dates defining when the certificate is valid. (to bind the functions in libnqsb-tls related to informations retrieval about the current peer)

It seems that those informations aren't currently exposed by the library, I had a chat with @hannesm that told me to open an issue, so here it is. :)

[hannesm]

these are Time.t from asn1-combinators, but should be Ptime.t (needs adjustment in asn1-combinator @pqwy) in the future