[services] add consts for secondary index names
Summary:
follow-up to the terraform diff that introduced two new GSIs. we will have to query by index name so we should make the names consts.
depends on D4647
Test Plan: cargo build
Reviewers: tomek, karol
Reviewed By: tomek
Subscribers: ashoat, adrian, atul, abosh
Differential Revision: https://phab.comm.dev/D4648