[identity] add sync-identity-search command

Summary:
This implements the sync-identity-search command in identity which syncs the opensearch search index with the latest user data from dynamodb.

Additionally verified on staging that:
Clear_index cleared the index by using an identity image that didn't clear and only restored
Scan works with 100+ items in DDB table by using 100+ test items on staging

Depends on D11124

Test Plan: Ran cargo run sync-identity-search on local and successfully confirmed search index was synced with latest dynamodb data

Reviewers: varun

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11126