Expose 'credentials' fetch option from Nuxeo client

Wen calling nuxeo endpoints using Nuxeo client we need to be able to set the credentials option of the underlying fetch library. For now, Nuxeo client is automatically set the credentials option to "include" when there is no authentication token set. In some situations, we need to set the 'credentials' option per request along with the oauth2 token to be able to receive cookies from Nuxeo. This is usefull for example to implement a revive http session action.