Add the ability to override the client ID which is used as the audience of the JWToken
Currently, the client ID obtained through the X-Dataporten-Clientid header is always used.
In order to make it easier to associate JWTokens with specific Kubernetes clusters, we want to make it possible to always use a specific client ID as the audience.
This commit allows the admin to use a specific client ID by setting the engine.dataporten.client_id config property. If no client ID is specified, we the X-Dataporten-Clientid header is used.