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]

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Co-authored-by: Maciej <[email protected]>