[identity] Configure identity search to return 20 search results

Summary:
This adds a size field to the opensearch search query with a size of 20 to match the current keyserver user search endpoint

Depends on D10996

Test Plan: Cargo run. Tested on local and staging

Reviewers: varun

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11162