[Tunnelbroker] add Platform column to device_token DDB table

Summary:
ENG-8903.

Platform is optional to support old clients which uploaded device token without platform.

Depends on D12937

Test Plan: Test inserting and getting device token with platform

Reviewers: bartek, tomek

Reviewed By: bartek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D12938