[Identity] DatabaseClient method to get user token

Summary: Adding a method to fetch a user's token from DynamoDB. The client will attempt to contruct and return a Token object from the DynamoDB item.

Test Plan: Successfully fetched tokens from DynamoDB with complete and incomplete data

Reviewers: jimpo, palys-swm, karol-bisztyga

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

Subscribers: ashoat, Adrian, yayabosh, atul

Differential Revision: https://phabricator.ashoat.com/D3957