client authentification with certificates

[daviian]

Instead of creating a user that is connected to a client it should be possible to create client certificates with with the existing cli cmd pagient-server admin create-client.

The command should return a private key and a signed certificate that can be used at the client to authenticate.
The client database entry could store the certificate signature for authentication and to establish the connection to the certificate.

Furthermore, we can then limit certain permissions to authentication type.
Clients authenticated by certificates can add/update patients whereas a user authenticated by username/password can only update the pager attribute of a patient.