Improve On_demand_assigner events (#4339)
title: Improving `on_demand_assigner` emitted events doc: - audience: Rutime User description: OnDemandOrderPlaced event that is useful for indexers to save data related to on demand orders. Check [discussion here](https://substrate.stackexchange.com/questions/11366/ondemandassignmentprovider-ondemandorderplaced-event-was-removed/11389#11389 ). Closes #4254 crates: [ 'runtime-parachain] --------- Co-authored-by: Maciej <[email protected]>