Endpoint for fetching party information for a given partyID

### Description

Backend issue for properly resolve #94

When loading an app instance where party is set we should in frontend ignore the party id in the cookie [frontend issue](https://github.com/Altinn/app-frontend-react/issues/1533)

Frontend receives the party id in the instance metadata, but has no endpoint to fetch the party information.

The parties controller needs to be extended with a get method for fetching party information for a given ID.
The authorization logic should be similar as in the {org}/{app}/api/authorization/parties/current endpoint

### Additional Information

_No response_

Start at the parties controller and compare its existing routes with the /{org}/{app}/api/authorization/parties/current endpoint and its authorization logic. Add a GET path that accepts a party ID and returns that party's information with equivalent authorization; done means the frontend can fetch the party identified in instance metadata.

Written by the indexing model from the issue text.