Prescium operator credential enrollment

The browser captures registration bytes only. Core derives the public key.

{"enrollment_challenge_ref":"b74546f25251415e83386330924dd5fc68d8e1e08cf358c48ba1d91424c80f30","origin":"https://mail.prescium.com","rp_id":"prescium.com","user_display_name":"Prescium Operator","user_handle_b64url":"zQeY5BDQNE-eENOFYIEhzzAntwoBtvF_ss4np_Sa8kk","user_name":"operator@prescium.com"}