Changes to `GET /v1/liquidity-pools/{pool_id}/activity` — the pool's operations (task 0491, issue #371), for classic and soroban pools alike (task 0374).

added the optional property data/items/event_index to the response with the 200 status

removed the required property data/items/application_order from the response with the 200 status

added the required property data/items/operation_index to the response with the 200 status