[terraform] [4/n] search_index_lambda terraform configuration options

Summary:
This adds support for search_index_lambda. This lambda consumes our DynamoDB stream for identity-users and then pushes these changes to OpenSearch

Depends on D9936
Depends on D10561

Test Plan: Verified terraform configuration on staging

Reviewers: varun, bartek, O1 Nix

Reviewed By: varun, O1 Nix

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9878