Consider adding a method to get a user-specific token

[phoglenix]

The DGAPI v2.0 supports getting tokens for purchases but does not have a way to get a token specific to a user.
A user-specific token could be useful for interacting with server-side methods, eg. a server-side analogue to listPurchases that returns already-verified purchase information, or a server-side analogue to getDetails so prices specific to the user could be server-side rendered.

This token might not be available in all store contexts, eg. Play Billing doesn't currently support this concept.

Ideally this token would be short-lived so it's not useful for fingerprinting.
This token could possibly be single-use, or take an input string to generate, so that it would be difficult to share among multiple clients.