Expose instance endpoints that uses organization number and social security number as an alternative to party id

### Description

As an external user of the application api's I would like to use organization number and/or social security number to avoid having to know about the Altinn internal partyId when working on instances.

### Additional Information

_No response_

### Tasks

_No response_

### Acceptance Criterias

_No response_

Start by locating the existing instance API endpoints that require an internal partyId and trace how party identity is handled. Clarify the endpoint shapes and lookup behavior for organization numbers and social security numbers; the work is done when the agreed alternatives are supported and verified for instance operations.

Written by the indexing model from the issue text.