zcaps: ability to configure expiry and clock skew

[llorllale]

This is a followup to task #101 which will leave the default values used by the underlying httpsignatures-go library:

• default expiry time: https://github.com/igor-pavlenko/httpsignatures-go#default-expires-seconds
• default "time gap" (aka clock skew): https://github.com/igor-pavlenko/httpsignatures-go#default-time-gap-for-expirescreated-time-verification