Add sanity check for credentials

[nichtsfrei]

As long as the openvas implementation is used it is not possible to define credentials with the same service type multiple times per target. This is because the credentials are getting exposed via a NASL script, that gets the preferences and exposes them as kb item, as an example:

krb5_login    = script_get_preference( "KRB5 login:", id:1 );
krb5_password = script_get_preference( "KRB5 password:", id:2 );
krb5_realm    = script_get_preference( "KRB5 realm:", id:3 );
krb5_kdc      = script_get_preference( "KRB5 kdc:", id:4 );

if( krb5_login )    set_kb_item( name:"KRB5/login_filled/0", value:krb5_login );
if( krb5_password ) set_kb_item( name:"KRB5/password_filled/0", value:krb5_password );
if( krb5_realm )    set_kb_item( name:"KRB5/realm_filled/0", value:krb5_realm );
if( krb5_kdc )      set_kb_item( name:"KRB5/kdc_filled/0", value:krb5_kdc );

exit( 0 );

This effectively limits the credential per target and credential service to one.

However neither the API documentation mentions that limitation nor does openvasd prevent adding multiple entries for the same service and target.

A scan with multiple entries per service

    "credentials": [
      {
        "service": "ssh",
        "up": {
          "username": "tesme",
          "password": "testme"
        },
        {
        "service": "ssh",
        "up": {
          "username": "testus",
          "password": "praktikus"
        }
    ],

Is accepted although it cannot work as the caller intended (try first, when not working try the second).

The expected behavior would be a 400 with a reasoning that ssh got defined twice.

Jira: SC-1212