Endpoint for fetching party information for a given partyID

[tjololo]

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

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