Expose helper functions for customer portal authentication

[brettflorio]

When using the JWT from the Foxy customer portal's authentication, a method to compare "now" against when the JWT expires (date_created + expires_in) is likely the first thing you'll need to do. We've done this ourselves in the content-protection script for Webflow, and in a number of other situations.

It's not a complicated function to write, but it's still something we should have a helper for. Per @pheekus, this may just be a matter of exposing what we're already doing in a way that's easily usable by a developer adding the portal to their site: https://github.com/Foxy/foxy-sdk/blob/main/src/customer/API.ts#L121-L136