[services] [1/n] Reorganize lambda function structs into query.rs and payload.rs

Summary:
This moves the structs from main.rs to their own separate files for code cleanliness.

I've created two new files: query.rs and payload.rs.

query.rs contains structs we use for constructing JSON for queries to our opensearch instance. payload.rs contains structs for dynamodb stream records.

Test Plan: Successfully compiled with cargo lambda

Reviewers: varun, bartek

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10788