[services] add GSI to users table for identity service

Summary: in order to look up userIDs by username or wallet address efficiently, we need a couple global secondary indexes. these will let us query by attribute instead of scanning the entire table.

Test Plan:
terraform validate and terraform fmt and terraform plan

note: learned that validate only checks the syntax. we should run plan to make sure DynamoDB-specific requirements are met

Reviewers: tomek, karol, max

Reviewed By: tomek, max

Subscribers: ashoat, adrian, atul, abosh

Differential Revision: https://phab.comm.dev/D4647