Currently we're only reading the known fields from profile_data:

https://github.com/realm/realm-object-store/blob/6b44209e6fcac0137e193c96444f93c50d184d06/src/sync/app.cpp#L619-L627

Instead, we should have a map of unknown fields because those may be valuable for end users. When a user configured the metadata fields for JWT auth, those get sent in the profileresponse.data field.

This is related to (https://github.com/realm/realm-js/pull/3302) that addresses it for Realm Web, but we need it fixed for other SDKs as well.