Instantiation: Allow bring-your-own `dialogId` in `dataValues`

When instantiating, it should be possible to bring your own dialogId instead of the app/sync-adapter automatically creating a new dialog upon instantiation. This would allow for more flexibility and control for app developers. Such a feature might need a flag or checks to allow/verify the supplied ID.

### Related discussions
- https://digdir.slack.com/archives/C08HYKJ8Z7F/p1781179134415809?thread_ts=1781162835.899149&cid=C08HYKJ8Z7F
- https://digdir-samarbeid.slack.com/archives/C069J6N7S00/p1781072705124899
- https://docs.altinn.studio/nb/dialogporten/reference/front-end/altinn-apps/#innstillinger-for-sync-adapter

### Current situation
* Dialogporten adapter supports config for not creating the dialog, but still keep it synchronised, if it is set in `DataValues` on the instance.
* There is currently no "approved" way to send the `dialog.id` when creating the instance without writing custom code that uses `IDataClient.UpdateDataValues`.

A long term solution will probably come when the app becomes able to take more responsibility with the new workflow engine. Short term we need an API to set `DataValues["dialog.id"]` using service owner token, either on instantiation or at a later point in time.

Start by reviewing the Dialogporten adapter configuration and the IDataClient.UpdateDataValues API mentioned in the issue, then trace instantiation and DataValues handling. Done should mean a supported way to provide dialog.id during or after instantiation, with the required validation and service owner token behavior clarified.

Written by the indexing model from the issue text.