[Identity] DatabaseClient method to put tokens in DDB

Summary:
This method, as explained in the diff title, lets us store user tokens in DynamoDB. Note that we convert the DateTime type to an RFC 3339 string. This seemed like the cleanest available option.

Depends on D3961

Test Plan: successfully put a token in DDB

Reviewers: jimpo, karol-bisztyga, palys-swm

Reviewed By: jimpo, karol-bisztyga

Subscribers: ashoat, Adrian, yayabosh, atul

Differential Revision: https://phabricator.ashoat.com/D3962