[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