Created/last_updated/last_update_by for open positions

[lukaslentner]

Is your feature request related to a problem? Please describe.
I need to understand on which data basis the result of a get open positions is based. Otherwise it is never clear which open position state belongs to which time where certain orders are already filled

Describe the solution you'd like
Add a created_at, updated_at field to allow users to understand if old http responses are still fresh. Additionally set the ID of the last account activity that contributed to the open position so that you know things are in sync.