[ENG-8345](https://linear.app/comm/issue/ENG-8345/add-ddb-table-for-tunnelbroker-to-persist-device-token)
Adding Index because when inserting we want to check if existing device token is present for any device - if so we want to delete it. Device token should be globally unique.
This is similar to existing [keyserver logic](https://github.com/CommE2E/comm/blob/be075d8192e3c0bf37f37083f13a8d0a4e69e9c6/keyserver/src/updaters/device-token-updaters.js#L9).