[terraform] create DDB table for device tokens

Summary:
ENG-8345

Adding Index because when inserting we want to check if the existing device token is present for any device - if so we want to delete it. Device tokens should be globally unique.
This is similar to existing keyserver logic.

Test Plan: Initialized localstack and used dynamodb-admin to verify table was created

Reviewers: bartek, marcin

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12516