[Identity] Add new types to handle tokens

Summary:
Our DatabaseClient will fetch user tokens from DynamoDB. We need a struct to store the data pertaining to a token.

We use the chrono crate to handle date/time

Test Plan: cargo build

Reviewers: jimpo, karol-bisztyga, palys-swm, ashoat

Reviewed By: jimpo, karol-bisztyga, palys-swm, ashoat

Subscribers: benschac, jonringer-comm, ashoat, Adrian, yayabosh, atul

Differential Revision: https://phabricator.ashoat.com/D3945