Add: privilege credential for SSH authenticated scans

[jjnicola]

What:
Add: privilege credential for SSH authenticated scans

After a successful authentication, the scanner can require privilege access for some test. In this case the privilege credential are used.
This means that the privilege username and password only make sense for SSH service and only if the regular username and password are given

Also, add a small fix for a parameter name in the openvasd/config.rs

Jira: SC-1025

Why:
was changed for UP credential and added for USK credential.

How:

Checklist:

• Tests
• PR merge commit message adjusted

[nichtsfrei]

Looks good, I like that the privileged information are duplicated instead of introducing a new credential type because it is making it easier to use.

An alternative approach could be to introduce a new struct:

struct PrivilegeInformation {
   username: String,
   password: String,
}

and include that as an option into the enum variants an option while annotating it with serde flatten:

...
  #[serde(flatten)]
  privelege: Option<PrivilegeInformation>,
...

that way we could ensure that username and password are written in the same way.

However since we just have two cases, it can be seen as optional :).