[services] add dynamodb tables for identity service to terraform

Summary: These tables already exist in DynamoDB but they should still be added here so we can manage them with Terraform.

Test Plan: ran terraform validate

Reviewers: geekbrother, palys-swm, karol-bisztyga

Reviewed By: geekbrother, palys-swm

Subscribers: ashoat, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4525