Add validity time option to the API

[paulgb]

Adds valid_for_seconds to the auth body request. We weren't actually parsing the request body before, so we do that now.

Introduces a newtype to avoid mixing up the current time and expiration times.

This is missing:

• JS SDK side
• Tests

If anyone has capacity feel free to take it from here and run with it, otherwise I'll pick it back up when I have a chance.