[services] [3/n] Rust opensearch indexing lambda

Summary:
A lambda written in Rust that consumes a dynamodb stream for identity-users and indexes users into the identity-search OpenSearch instance.

Depends on D9877

Test Plan: Successfully built using cargo lambda and deployed with terraform. Confirmed manually the lambda is syncing the stream.

Reviewers: varun, bartek

Reviewed By: varun, bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9936